Samacheer Kalvi Class 9 Science Solutions Chapter 9 Universe

I. Choose the correct answer:

 

Question 1. Who proposed the heliocentric model of the universe?
(a) Tycho Brahe
(b) Nicolaus Copernicus
(c) Ptolemy
(d) Archimedes
Answer: (b) Nicolaus Copernicus
In simple words: Nicolaus Copernicus was the first to suggest that the Earth and other planets revolve around the Sun, not the Earth itself. This idea changed how people understood our solar system.

🎯 Exam Tip: Remember key figures like Copernicus for their groundbreaking contributions to astronomy, especially for models of the solar system.

 

Question 2. Which of the following is not a part of outer solar system?
(a) Mercury
(b) Saturn
(c) Uranus
(d) Neptune
Answer: (a) Mercury
In simple words: The outer solar system includes the gas giant planets that are far from the Sun. Mercury is a rocky planet close to the Sun, so it is part of the inner solar system.

🎯 Exam Tip: Classify planets into inner (rocky) and outer (gas giants) solar system groups to quickly identify exceptions.

 

Question 3. Ceres is a ..............
(a) Meteor
(b) Star
(c) Planet
(d) Asteroid
Answer: (d) Asteroid
In simple words: Ceres is a very large asteroid that orbits between Mars and Jupiter. It is also called a dwarf planet because of its size and shape.

🎯 Exam Tip: Understand the difference between celestial bodies like asteroids, planets, and stars based on their characteristics and orbits.

 

Question 4. The period of revolution of planet A around the Sun is 8 times that of planet B. How many times is the distance of planet A as great as that of planet B?
(a) 4
(b) 5
(c) 2
(d) 3
Answer: (a) 4
In simple words: Kepler's Third Law links a planet's orbital period and its distance from the Sun. If the period is 8 times longer, the distance is 4 times greater.

🎯 Exam Tip: This question relates to Kepler's Third Law (Law of Harmonies), where the square of the orbital period is proportional to the cube of the semi-major axis (distance). If \( T_A = 8 T_B \), then \( T_A^2 = (8 T_B)^2 = 64 T_B^2 \). So, \( R_A^3 = 64 R_B^3 \), which means \( R_A = \sqrt[3]{64} R_B = 4 R_B \).

 

Question 5. The Big Bang occurred .............. years ago.
(a) 13.7 billion
(b) 15 million
(c) 15 billion
(d) 20 million
Answer: (a) 13.7 billion
In simple words: Scientists believe the universe started with a huge explosion called the Big Bang about 13.7 billion years ago. This event created everything we see today.

🎯 Exam Tip: Know the approximate age of the universe according to the Big Bang theory. "Billion" is a key word here, not "million".

 

II. Fill in the blanks :

 

Question 1. The speed of Sun in km/s is ..............
Answer: 250 km/s
In simple words: The Sun moves very fast through space, traveling at 250 kilometers every second. This motion is part of how our galaxy spins.

🎯 Exam Tip: Remember specific astronomical values like the Sun's speed; they are common factual questions.

 

Question 2. The rotational period of the Sun near its poles is ..............
Answer: 36 days
In simple words: Unlike Earth, the Sun does not rotate at the same speed everywhere. Near its top and bottom (poles), it takes 36 days to spin once.

🎯 Exam Tip: Note that the Sun has differential rotation, meaning its rotation period varies with latitude, which is different from solid planets.

 

Question 3. India's first satellite is ..............
Answer: Aryabhatta
In simple words: Aryabhatta was the very first satellite launched by India. It marked a big step for India in space science.

🎯 Exam Tip: Historical firsts in science and technology are often tested, so remember key names and dates like Aryabhatta.

 

Question 4. The third law of Kepler is also known as the Law of ..............
Answer: Harmonics
In simple words: Kepler's third law connects a planet's orbital time to its distance from the Sun. It is called the Law of Harmonies because it describes a consistent relationship.

🎯 Exam Tip: Be sure to know all three of Kepler's laws, including their common names, as they are fundamental to planetary motion.

 

Question 5. The number of planets in our Solar System is ..............
Answer: 8
In simple words: Our solar system has eight main planets that orbit the Sun. These are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

🎯 Exam Tip: Ensure you remember the current number of recognized planets in our solar system after Pluto was reclassified.

 

III. State whether true or false. If false, correct the statement:

 

Question 1. ISS is a proof for international cooperation.
Answer: True.
In simple words: The International Space Station (ISS) shows how different countries can work together on big science projects. Many nations helped build and operate it.

🎯 Exam Tip: When answering true/false questions, always understand the core concept being tested, in this case, the collaborative nature of the ISS.

 

Question 2. Halley's comet appears after nearly 67 hours.
Answer: False.
Correct statement: Halley's comet appears after nearly 76 years.
In simple words: Halley's comet takes a very long time to orbit the Sun and be seen again, not just a few hours. It comes back around every 76 years.

🎯 Exam Tip: For "false" statements requiring correction, provide the accurate fact clearly and concisely.

 

Question 3. Satellites nearer to the Earth should have lesser orbital velocity.
Answer: False.
Correct statement: Satellites nearer to the Earth should have faster orbital velocity.
In simple words: Satellites that are closer to Earth need to move faster to stay in orbit. If they move too slowly, Earth's gravity will pull them down.

🎯 Exam Tip: Remember that orbital speed is inversely related to orbital radius for stable orbits; closer orbits require higher speeds to counteract stronger gravity.

 

Question 4. Mars is called the red planet.
Answer: True.
In simple words: Mars gets its nickname "red planet" from the iron oxide (rust) on its surface, which gives it a reddish appearance.

🎯 Exam Tip: Common names or nicknames for planets are often related to their visible characteristics; linking them helps recall.

 

IV. Answer briefly :

 

Question 1. What is the solar system?
Answer: The solar system is made up of the Sun and all the celestial bodies that revolve around it. These bodies include planets, comets, asteroids, and meteors, all held together by gravity. Our solar system is a small part of a much larger galaxy, the Milky Way.
In simple words: The solar system is the Sun and everything that orbits it, like planets, comets, and asteroids.

🎯 Exam Tip: Define the solar system by listing its main components and the central role of the Sun.

 

Question 2. Define orbital velocity.
Answer: Orbital velocity is the horizontal speed needed for a satellite to stay in a circular path around a planet at a certain height. This speed makes sure the satellite does not fall back to the planet. It is crucial for keeping satellites in space.
In simple words: Orbital velocity is the speed a satellite needs to move sideways to stay in a circle around a planet.

🎯 Exam Tip: Emphasize "horizontal velocity" and "determined height" when defining orbital velocity, as these are key components.

 

Question 3. Define time period of a satellite.
Answer: The time period of a satellite is the total time it takes for the satellite to complete one full orbit around the Earth. This period is related to the distance covered and the orbital velocity, which can be expressed by the formula: \( T = \frac{2\pi r}{v} \), where \( T \) is the time period, \( r \) is the orbital radius, and \( v \) is the orbital velocity. This period is crucial for tracking and using satellites.
In simple words: The time period is how long a satellite takes to go around the Earth one full time.

🎯 Exam Tip: Include the formula for the time period \( (T = \frac{2\pi r}{v}) \) if asked to define it, as it shows a deeper understanding.

 

Question 4. What is the satellite? What are the two types of satellites?
Answer: A satellite is any object that moves in orbit around a larger celestial body, such as a planet. Satellites can be categorized into two main types: 1. Natural satellites: These are celestial bodies like the Moon, which orbits Earth. 2. Man-made satellites: These are artificial objects launched by humans, such as communication satellites or the International Space Station. Both types play important roles in our understanding and use of space.
In simple words: A satellite is anything that circles a planet. The two types are natural ones like our Moon, and man-made ones that we send into space.

🎯 Exam Tip: Clearly distinguish between natural and artificial satellites with relevant examples for each type.

 

Question 5. Write a note on the inner planets.
Answer: The inner planets are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. They are known as terrestrial or rocky planets because they have solid surfaces, like Earth's rocky crust. These planets are closer together compared to the outer planets and share similar ways their insides, surfaces, and atmospheres were formed. Earth is a good example for understanding all inner planets.
In simple words: Inner planets are Mercury, Venus, Earth, and Mars. They are close to the Sun, made of rock, and have solid surfaces.

🎯 Exam Tip: When describing inner planets, remember to mention their names, composition (rocky/terrestrial), and proximity to the Sun.

 

Question 6. Write about comets.
Answer: Comets are icy, dusty objects that travel around the Sun in highly stretched-out (elliptical) paths.

• They are made of lumps of dust and ice.
• Comets take a very long time to complete one orbit.
• As a comet gets closer to the Sun, its ice turns into gas, creating a bright head and a long tail.
• The tail of very large comets can be as long as 160 million kilometers.
• Some comets, like Halley's comet, come back regularly, appearing every 76 years.

These celestial bodies offer clues about the early solar system.
In simple words: Comets are space objects made of ice and dust that orbit the Sun in long, oval paths. They grow a bright tail when they get close to the Sun.

🎯 Exam Tip: List key characteristics of comets: composition (ice/dust), orbital shape (elliptical), behavior near the Sun (tail formation), and periodicity (e.g., Halley's Comet).

 

Question 7. State Kepler's laws.
Answer: In the early 1600s, Johannes Kepler developed three fundamental laws that describe how planets move around the Sun.

• First Law – The Law of Ellipses: This law states that planets orbit the Sun in paths shaped like ellipses, not perfect circles. The Sun is located at one of the two focal points of this ellipse.
• Second Law – The Law of Equal Areas: An imaginary line connecting the Sun to a planet sweeps out equal areas in equal amounts of time. This means a planet moves faster when it is closer to the Sun and slower when it is farther away.
• Third Law – The Law of Harmonies: The ratio of the square of a planet's orbital period (the time it takes to complete one orbit) to the cube of its average distance from the Sun is the same for all planets. This law helps us compare the orbits of different planets.

These laws are crucial for understanding celestial mechanics.
In simple words: Kepler's laws explain how planets move around the Sun. The first law says orbits are oval-shaped. The second says planets move faster when closer to the Sun. The third links a planet's orbit time to its distance from the Sun.

🎯 Exam Tip: Clearly state and briefly explain each of Kepler's three laws, remembering their names and what each law describes about planetary motion.

 

Question 8. What factors have made life on Earth possible?
Answer: Several important factors make life possible on Earth:

1. Right distance from the Sun: Earth is at a perfect distance, allowing for moderate temperatures that support liquid water.
2. Right temperature: The Earth's temperature range is suitable for living organisms to survive and thrive.
3. Presence of water: Water is essential for all known life forms and covers a large part of Earth's surface.
4. Suitable atmosphere: Earth's atmosphere contains gases like oxygen that living things breathe and protects from harmful radiation.
5. Blanket of ozone: The ozone layer in the atmosphere blocks dangerous ultraviolet (UV) rays from the Sun.

These combined conditions create a unique environment for life.
In simple words: Life is possible on Earth because it is just the right distance from the Sun, has good temperatures, plenty of water, air to breathe, and an ozone layer for protection.

🎯 Exam Tip: List specific, interconnected factors that contribute to Earth's habitability, focusing on its unique atmospheric and positional characteristics.

 

V. Answer in detail:

 

Question 1. Give an account of all the planets in the solar system.
Answer: The solar system has eight planets, which are divided into two main groups based on their characteristics and distance from the Sun.
Inner Planets (Terrestrial/Rocky Planets):The first four planets are Mercury, Venus, Earth, and Mars. They are relatively close to the Sun and to each other. These planets are called inner planets because they are found in the inner part of the solar system. They have solid, rocky surfaces and crusts, similar to Earth. Their internal structures, surfaces, and atmospheres were formed in a comparable manner, making Earth a good model for understanding them. These planets do not have rings.
Outer Planets (Gas Giants/Gaseous Planets):Beyond the inner planets lies the outer solar system, where the planets are much more spread out. The four large outer planets are Jupiter, Saturn, Uranus, and Neptune. These planets are made mostly of hydrogen, helium, and other gases in vast amounts, giving them very dense atmospheres. They are known as gas giants because they do not have solid surfaces like the inner planets. A key feature of the outer planets, including Jupiter, Saturn, Uranus, and Neptune, is that they all have rings, which are made up of tiny pieces of rock covered in ice. For example, the distance between Saturn and Uranus is about 20 times greater than that between Earth and Mars. All these planets follow elliptical orbits around the Sun.
In simple words: Our solar system has eight planets. The four inner ones (Mercury, Venus, Earth, Mars) are rocky and close to the Sun. The four outer ones (Jupiter, Saturn, Uranus, Neptune) are huge gas planets with rings, far from the Sun.

🎯 Exam Tip: Organize your answer by categorizing planets into inner and outer groups, describing their composition, location, and key features like rings or solid surfaces.

 

Question 2. Discuss the benefits of ISS.
Answer: The International Space Station (ISS) has brought many benefits to Earth through its research and technological developments:

1. Water Purification Efforts: Techniques developed for the ISS's water recovery system (WRS) and oxygen generation system (OGS) have led to advanced water treatment methods. These methods help provide clean water in areas facing water scarcity on Earth.
2. Eye-tracking Technology: Eye-tracking technology, refined for use on the ISS, is now used in laser eye surgeries. It tracks eye position very accurately. This technology also helps people with limited movement or speech communicate more easily.
3. Robotic Arms and Surgeries: Robotic arms, similar to those used on the ISS, assist surgeons. They provide significant help in delicate procedures, such as removing difficult tumors and taking biopsies with high precision.
4. Other Applications: Beyond these, ISS research has contributed to many other fields. This includes the creation of better vaccines and improved methods for breast cancer detection and treatment.

The ISS continues to be a hub for scientific breakthroughs that improve life on Earth.
In simple words: The ISS helps us in many ways: it provides better ways to clean water, uses eye-tracking for surgery and communication, improves robotic tools for doctors, and aids in developing vaccines and cancer detection.

🎯 Exam Tip: When discussing benefits, focus on concrete applications and technologies that have directly impacted life on Earth, linking ISS research to practical uses.

 

Question 3. Write a note on orbital velocity.
Answer: Orbital velocity is the specific horizontal speed that a satellite must have to maintain a circular orbit around a planet at a certain altitude. If the velocity is too low, the satellite will fall back to Earth; if it's too high, it will escape Earth's gravity.
Many artificial satellites are launched into Earth's orbit. For example, India launched its first satellite, Aryabhatta, on April 19, 1975, following the launch of Sputnik in 1956. These satellites typically orbit at a height of a few hundred kilometers where air friction is very small. To achieve orbit, a rocket carries the satellite to the desired height and releases it horizontally with the necessary high velocity.
The orbital velocity of a satellite changes depending on its altitude above Earth. Satellites closer to Earth need a faster orbital velocity. For instance, at an altitude of 200 kilometers, an orbital velocity of over 27,400 kilometers per hour is needed. This speed allows the satellite to complete one revolution in about 24 hours. Because Earth also rotates in 24 hours, a satellite in this type of orbit stays over the same spot on Earth's surface all the time, known as a 'geostationary' orbit.
The orbital velocity \( (v) \) can be calculated using the formula:
\( v = \sqrt{\frac{GM}{(R+h)}} \)
where: \( G \) = Gravitational constant \( (6.673 \times 10^{-11} \text{Nm}^2\text{kg}^{-2}) \) \( M \) = Mass of the Earth \( (5.972 \times 10^{24}\text{kg}) \) \( R \) = Radius of the Earth \( (6371 \text{ km}) \) \( h \) = Height of the satellite from the surface of the Earth.
Understanding orbital velocity is vital for launching and maintaining satellites.
In simple words: Orbital velocity is the exact speed a satellite needs to move sideways to stay in a circular path around Earth. It depends on how high the satellite is, with closer satellites needing to move faster. Some satellites stay over the same spot, called geostationary. The speed is found using a formula that includes Earth's mass and gravity.

🎯 Exam Tip: Fully explain orbital velocity, including its definition, factors affecting it (altitude), examples, the concept of geostationary orbits, and the mathematical formula with its variables.

 

VI. Conceptual Questions

 

Question 1. Why do some stars appear blue and some red?
Answer: Stars appear in different colors because their temperature varies.

1. Hot stars, which burn at very high temperatures, tend to look white or blue.
2. Cooler stars, which are not as hot, appear orange or red.

This color change is a natural part of how stars give off light. For example, our Sun is a yellow star, meaning it has a medium temperature compared to other stars.
In simple words: Stars look different colors based on how hot they are. Very hot stars are blue or white, while cooler stars are orange or red.

🎯 Exam Tip: Link star color directly to temperature: blue/white for hot, red/orange for cool. This is a common and important astronomical concept.

 

Question 2. How is a satellite maintained in nearly circular orbit?
Answer: An artificial satellite is kept in a nearly circular orbit by carefully controlling its launch and initial velocity.
First, the satellite is sent to a specific height, usually a few hundred kilometers above Earth. At this height, there is very little air, so friction is almost zero.
Next, the satellite is released horizontally from the rocket with a very high speed. This horizontal speed is crucial. If the satellite moves at the correct high velocity, it will continuously "fall" around the Earth without hitting the ground. This constant falling motion keeps it in a continuous, nearly circular path. This balance between gravity pulling it down and its horizontal speed keeps it stable in orbit.
In simple words: Satellites stay in orbit by being launched very high up and then pushed sideways at a very fast speed. This fast side movement makes them fall around the Earth instead of falling to the Earth.

🎯 Exam Tip: Highlight the importance of negligible air friction at high altitudes and the precise horizontal velocity needed to achieve and maintain orbit.

 

Question 3. Why are some satellites called geostationary?
Answer: Some satellites are called geostationary because they stay in a fixed position relative to a specific point on Earth's surface. This happens when a satellite orbits Earth at a particular altitude and speed, completing one revolution in exactly 24 hours. Since Earth also rotates once every 24 hours, the satellite appears to hover over the same geographical location all the time. This makes them ideal for communication and weather monitoring because their ground antennas don't need to move.
In simple words: Geostationary satellites are those that seem to stay in one spot in the sky because they orbit the Earth at the same speed the Earth spins.

🎯 Exam Tip: Explain that a geostationary orbit's period matches Earth's rotation period, making the satellite appear stationary from the ground. This is a key characteristic for these satellites.

 

Question 4. A man weighing 60 kg on the Earth will weigh 1680 kg in the Sun. Why?
Answer: A man with a mass of 60 kg would experience a much greater weight on the Sun compared to Earth. This is because the Sun has a gravitational acceleration about 28 times stronger than Earth's.
Given: Mass of the man \( = 60 \text{ Kg} \) Gravitational acceleration on the Sun \( (g_{\text{Sun}}) = 274.13 \text{ m/s}^2 \) (approximately 28 times Earth's gravity)
The weight \( (W) \) of an object is calculated as mass \( (m) \) multiplied by gravitational acceleration \( (g) \).
\( W = m \times g \)
On the Sun, the man's weight would be:
\( W_{\text{Sun}} = 60 \text{ kg} \times 274.13 \text{ m/s}^2 = 16,447.8 \text{ N} \)
This massive difference in weight is solely due to the Sun's enormously strong gravity. If we were to incorrectly express this weight as an equivalent mass on Earth, it would be a very large number, but scientifically, weight is a force measured in Newtons.
In simple words: A man would weigh much more on the Sun because the Sun's gravity is far, far stronger than Earth's. Even though his mass stays 60 kg, the pull of gravity on him is huge, making his weight very high.

🎯 Exam Tip: Clearly state that weight is a force \( (W=mg) \) and explain that the difference in weight is due to the varying gravitational acceleration of celestial bodies. Distinguish between mass and weight.

 

VII. Numerical Problems

 

Question 1. Calculate the speed with which a satellite moves if it is at a height of 36,000 km from the Earth's surface and has an orbital period of 24 hr (Take R = 6370 km). [Hint: Convert hr into seconds before doing the calculation)
Answer: To calculate the speed of the satellite, we use the formula for orbital velocity and the time period.
Given: Height of the satellite, \( h = 36,000 \text{ km} = 36,000 \times 10^3 \text{ m} \) Radius of the Earth, \( R = 6370 \text{ km} = 6370 \times 10^3 \text{ m} \) Orbital period, \( T = 24 \text{ hr} = 24 \times 3600 \text{ s} = 86400 \text{ s} \)
We know the formula for orbital velocity:
\( v = \frac{2\pi (R+h)}{T} \)
First, calculate the orbital radius \( (r = R+h) \):
\( r = (6370 + 36000) \text{ km} = 42370 \text{ km} = 42370 \times 10^3 \text{ m} \)
Now, substitute the values into the velocity formula:
\( v = \frac{2 \times 3.14 \times (42370 \times 10^3 \text{ m})}{86400 \text{ s}} \)
\( v = \frac{266183.6 \times 10^3 \text{ m}}{86400 \text{ s}} \)
\( v \approx 3.0808 \times 10^3 \text{ m/s} \)
\( v \approx 3080.8 \text{ m/s} \) (or) \( 3.08 \text{ km/s} \)
The speed of the satellite is approximately \( 3080.8 \text{ m/s} \). The provided solution calculates \( v \) using \( v = \sqrt{\frac{GM}{(R+h)}} \) and then separately calculates \( T \). Let's follow the solution's first part for \( v \) and then the hint regarding \( T \). The provided OCR uses the formula \( v=\sqrt{\frac{GM}{(R+h)}} \) for the initial calculation but then also gives \( T = 2\pi(R+h)/\nu \). I will combine both approaches and try to derive the velocity from the orbital period. The solution provided uses a direct \( v \) calculation first, then a \( T \) calculation. The question specifically asks for speed *given* the period. So I should use the period to find speed.
Let's re-evaluate based on the given solution's calculation flow and my understanding of the question (speed *given* period). The provided solution's calculation of velocity uses \( v=\sqrt{\frac{GM}{(R+h)}} \) and finds \( v \approx 3067 \text{ m/s} \). Then it calculates the period \( T = \frac{2\pi(R+h)}{v} \). The question gives us \( T = 24 \text{ hr} \). So we should use this \( T \) to find \( v \).
Given: Height of the satellite, \( h = 36,000 \text{ km} = 36,000 \times 10^3 \text{ m} \) Radius of the Earth, \( R = 6370 \text{ km} = 6370 \times 10^3 \text{ m} \) Orbital period, \( T = 24 \text{ hr} = 24 \times 3600 \text{ s} = 86400 \text{ s} \)
Orbital radius \( r = R+h = 6370 \text{ km} + 36000 \text{ km} = 42370 \text{ km} = 42370 \times 10^3 \text{ m} \)
The formula for orbital velocity \( v \) when period \( T \) and radius \( r \) are known is:
\( v = \frac{2\pi r}{T} \)
Substitute the values:
\( v = \frac{2 \times 3.14 \times (42370 \times 10^3 \text{ m})}{86400 \text{ s}} \)
\( v = \frac{266183.6 \times 10^3 \text{ m}}{86400 \text{ s}} \)
\( v \approx 3080.82 \text{ m/s} \)
So, the speed of the satellite is approximately \( 3080.82 \text{ m/s} \) or \( 3.08 \text{ km/s} \).
The provided solution on page 12 directly calculates \( v \) using \( \sqrt{\frac{GM}{(R+h)}} \) and gets \( 3067 \text{ m/s} \). The values of G and M are given. Let's use the method presented in the source for consistency, even if the question implied using the period. The source calculates v first.
Given: Height of the satellite, \( h = 36,000 \text{ km} = 36000 \times 10^3 \text{ m} \) Radius of the Earth, \( R = 6370 \text{ km} = 6370 \times 10^3 \text{ m} \) Gravitational constant, \( G = 6.673 \times 10^{-11} \text{ Nm}^2\text{kg}^{-2} \) Mass of the Earth, \( M = 5.972 \times 10^{24} \text{ kg} \)
We know the formula for orbital velocity:
\( v = \sqrt{\frac{GM}{(R+h)}} \)
First, calculate \( (R+h) \):
\( R+h = (6370 + 36000) \text{ km} = 42370 \text{ km} = 42370 \times 10^3 \text{ m} \)
Now, substitute the values into the formula:
\( v = \sqrt{\frac{(6.673 \times 10^{-11}) \times (5.972 \times 10^{24})}{(42370 \times 10^3)}} \)
\( v = \sqrt{\frac{39.851996 \times 10^{13}}{42370 \times 10^3}} \)
\( v = \sqrt{\frac{3.985 \times 10^{14}}{4.237 \times 10^{7}}} \)
\( v = \sqrt{0.9405 \times 10^7} \)
\( v = \sqrt{9.405 \times 10^6} \)
\( v \approx 3066.76 \text{ m/s} \)
So, the speed of the satellite is approximately \( 3067 \text{ m/s} \) or \( 3.067 \text{ km/s} \). This calculation directly matches the source's result. This speed is constant for a stable orbit.
In simple words: To find how fast the satellite moves, we use a special formula that includes Earth's mass, gravity, and the satellite's height. After putting in all the numbers, we find the satellite travels at about 3067 meters per second, or roughly 3 kilometers every second.

🎯 Exam Tip: When solving numerical problems, always write down the given values and the formula you are using. Remember to convert units (like km to m, or hr to s) before calculation to avoid errors.

 

Question 2. At an orbital height of 400 km, find the orbital period of the satellite.
Answer: To find the orbital period of the satellite at 400 km height, we first need to calculate its orbital velocity.
Given: Orbital height, \( h = 400 \text{ km} = 400 \times 10^3 \text{ m} \) Radius of the Earth, \( R = 6371 \text{ km} = 6371 \times 10^3 \text{ m} \) Gravitational constant, \( G = 6.673 \times 10^{-11} \text{ Nm}^2\text{kg}^{-2} \) Mass of the Earth, \( M = 5.972 \times 10^{24} \text{ kg} \)
First, calculate the orbital radius \( r = R+h \):
\( r = (6371 + 400) \text{ km} = 6771 \text{ km} = 6771 \times 10^3 \text{ m} \)
Now, calculate the orbital velocity \( v \) using the formula:
\( v = \sqrt{\frac{GM}{(R+h)}} \)
\( v = \sqrt{\frac{(6.673 \times 10^{-11}) \times (5.972 \times 10^{24})}{(6771 \times 10^3)}} \)
\( v = \sqrt{\frac{39.851996 \times 10^{13}}{6771 \times 10^3}} \)
\( v = \sqrt{\frac{3.985 \times 10^{14}}{6.771 \times 10^6}} \)
\( v = \sqrt{0.5885 \times 10^8} \)
\( v = \sqrt{58.85 \times 10^6} \)
\( v \approx 7.671 \times 10^3 \text{ m/s} = 7671 \text{ m/s} \)
Now that we have the orbital velocity, we can find the time period \( T \) using the formula:
\( T = \frac{2\pi r}{v} \)
\( T = \frac{2 \times 3.14 \times (6771 \times 10^3 \text{ m})}{7671 \text{ m/s}} \)
\( T = \frac{42598.68 \times 10^3 \text{ m}}{7671 \text{ m/s}} \)
\( T \approx 5553.2 \text{ s} \)
Convert seconds to minutes:
\( T = \frac{5553.2}{60} \text{ minutes} \approx 92.55 \text{ minutes} \)
The orbital period of the satellite at 400 km height is approximately \( 5553.2 \text{ seconds} \) or \( 92.55 \text{ minutes} \). This indicates a very fast orbit, typical for low-Earth satellites. The provided solution shows 5540 s and 92.3 min. My calculation is very close.
In simple words: To find how long it takes for a satellite to go around Earth at 400 km height, we first calculate its speed using Earth's mass and gravity. Then, we use that speed and the orbit's size to find the time it takes. It turns out to be about 92.5 minutes for one full circle.

🎯 Exam Tip: For problems involving both orbital velocity and period, it's often a two-step calculation: first find velocity, then use it to find the period. Pay attention to unit conversions throughout the process.

 

Intext Activities

 

ACTIVITY - 1

 

ACTIVITY - 2

 

Activity - 3

Watch the sky in the early morning. Do you see any planet? What is its name? Find out with the help of your teachers.
Aim:
1. Most of the planets cannot be seen in normal sky.
2. Few planets can be seen during some specific period in the night sky.
3. Mercury, closest to the sun can be seen from the earth.
4. Venus is also seen few days in a year.
5. Uranus may be also seen on odd days in October.

🎯 Exam Tip: When doing observation activities, note down the exact dates and times you observed, as planetary visibility changes over time.

 

Activity - 4

Prepare a list of Indian satellites from Aryabhatta to the latest along with their purposes.
Answer:

In simple words: India has launched many satellites for different jobs, from studying Earth to helping with communication and education. Each satellite, like Aryabhatta or Chandrayaan, has a special task.

🎯 Exam Tip: Memorize the first few Indian satellites and their primary purposes, as they are common general knowledge questions.

 

9th Science Guide Universe Additional Important Questions and Answers

I. Choose the correct answer:

 

Question 1. The Geocentric model of universe was proposed by
(a) Tyago Brane
(b) Kepler
(c) Ptolemy
(d) Copernicus
Answer: (c) Ptolemy
In simple words: The Geocentric model is an old idea that says Earth is the center of the universe, and Ptolemy was a key person who supported this view.

🎯 Exam Tip: Remember that "Geocentric" refers to Earth at the center, while "Heliocentric" refers to the Sun at the center.

 

Question 2. The total area of observable universe is about light-years.
(a) 9.3 billion
(b) 93 billion
(c) 93 million
(d) 9.3 million
Answer: (b) 93 billion
In simple words: The part of the universe we can see from Earth is huge, stretching about 93 billion light-years across. This is because light from faraway objects takes a very long time to reach us.

🎯 Exam Tip: A light-year is a unit of distance, not time. It is the distance light travels in one year.

 

Question 3. All the atoms together only make up percent of universe.
(a) 3
(b) 4
(c) 5
(d) 2
Answer: (b) 4
In simple words: All the regular matter, like atoms that make up stars, planets, and us, accounts for only about 4% of the entire universe. The rest is dark matter and dark energy, which we cannot see directly.

🎯 Exam Tip: Understand the difference between regular (baryonic) matter, dark matter, and dark energy, and their approximate percentages in the universe.

 

Question 4. The milky way galaxy is in shape.
(a) Elliptical
(b) Circular
(c) Spiral
(d) Irregular
Answer: (c) Spiral
In simple words: Our home galaxy, the Milky Way, has a beautiful spiral shape, with long arms swirling around a central bulge. Most galaxies have either a spiral, elliptical, or irregular shape.

🎯 Exam Tip: Be able to identify and describe the main types of galaxies by their shapes.

 

Question 5. Stars are built by gases.
(a) Hydrogen
(b) Helium
(c) Nitrogen
(d) Oxygen
Answer: (a) Hydrogen
In simple words: Stars are mainly made of hydrogen gas, which they burn through a process called nuclear fusion to produce light and heat. Over time, this hydrogen changes into helium and other heavier elements.

🎯 Exam Tip: Recall that hydrogen is the most abundant element in the universe and the primary fuel for stars.

 

Question 6. The process that takes place in the Sun is
(a) Nuclear Fission
(b) Spallation
(c) Nuclear Fusion
(d) None of the options
Answer: (c) Nuclear Fusion
In simple words: The Sun creates its immense energy by smashing small atoms, mainly hydrogen, together to form larger atoms, like helium. This process, called nuclear fusion, releases a massive amount of energy, making the Sun shine.

🎯 Exam Tip: Differentiate between nuclear fission (splitting atoms) and nuclear fusion (combining atoms) and know which process powers stars.

 

Question 7. The energy of the sun is radiated in the form of Rays.
(a) Ultraviolet
(b) Infrared
(c) neutron radiation
(d) X-rays
Answer: (b) Infrared
In simple words: The Sun sends out energy in many forms, and infrared rays are a significant part of this. Infrared radiation is what we feel as heat from the sun.

🎯 Exam Tip: The Sun emits a full spectrum of electromagnetic radiation, including visible light, ultraviolet, and infrared rays.

 

Question 8. The colour is the most intense colour in solar radiation.
(a) blue
(b) red
(c) violet
(d) yellow
Answer: (d) yellow
In simple words: The Sun gives off light in many colors, but it shines most brightly in the yellow-green part of the spectrum. This is why our Sun often looks yellow to us, especially when it is higher in the sky.

🎯 Exam Tip: The perceived color of the Sun can vary based on atmospheric conditions and the time of day, but its peak emission is in the yellow-green range.

 

Question 9. If the distance of the planet from the sun increases, the period of revolution
(a) increases
(b) decreases
(c) remains the same
(d) a & b
Answer: (a) increases
In simple words: According to Kepler's Third Law, planets that are farther away from the Sun take longer to complete one full trip around it. This is why outer planets like Neptune have much longer years than inner planets like Earth.

🎯 Exam Tip: Kepler's laws of planetary motion are fundamental; remember that orbital period increases with orbital radius.

 

Question 10. Which among the planets has less length of a day?
(a) Mercury
(b) Uranus
(c) Jupiter
(d) Saturn
Answer: (c) Jupiter
In simple words: Jupiter spins very fast on its axis, completing a rotation in less than 10 hours, which gives it the shortest day among all planets in our solar system. This rapid spin also causes its distinctive flattened shape at the poles.

🎯 Exam Tip: Jupiter's large size and rapid rotation are key characteristics to remember. Also recall that gas giants generally rotate faster than rocky planets.

 

Question 11. The hottest planet on the solar system is
(a) Venus
(b) Mercury
(c) Mars
(d) Earth
Answer: (a) Venus
In simple words: Even though Mercury is closer to the Sun, Venus is the hottest planet because its thick atmosphere traps a lot of heat, creating a strong greenhouse effect. This makes its surface incredibly hot, hotter than Mercury.

🎯 Exam Tip: Do not assume the closest planet to the Sun is the hottest; atmospheric composition plays a crucial role in planetary temperatures.

 

Question 12. Which planet is known as Red Planet?
(a) Venus
(b) Saturn
(c) Mars
(d) Neptune
Answer: (c) Mars
In simple words: Mars is called the Red Planet because its surface is covered in iron-rich dust that rusts, giving the planet its reddish-orange color. This iron oxide is similar to rust on Earth.

🎯 Exam Tip: Associate Mars with its distinctive red color, caused by iron oxide (rust) on its surface.

 

Question 13. The number of moons present in Saturn is
(a) 20
(b) 30
(c) 50
(d) 60
Answer: (d) 60
In simple words: Saturn has many moons, with 60 being a commonly cited number for its major satellites. The actual count can change as new, smaller moons are discovered around gas giants like Saturn.

🎯 Exam Tip: Be aware that the number of moons for gas giants is quite high and can be updated as space exploration continues.

 

Question 14. The planet having the longest summers and winters is
(a) Saturn
(b) Neptune
(c) Uranus
(d) Jupiter
Answer: (c) Uranus
In simple words: Uranus has extremely long seasons, each lasting about 21 Earth years, because it rotates almost on its side. This extreme tilt causes one pole to face the Sun for very long periods, leading to extended summers and winters.

🎯 Exam Tip: Remember Uranus's unique axial tilt, which causes its unusual and very long seasonal cycles.

 

Question 15. Windiest planet in the solar system
(a) Neptune
(b) Uranus
(c) Saturn
(d) Jupiter
Answer: (a) Neptune
In simple words: Neptune is known for having the strongest winds in our solar system, with speeds that can reach over 2,000 kilometers per hour. These powerful winds create large storms and features like the Great Dark Spot.

🎯 Exam Tip: Associate Neptune with its very strong winds and dynamic atmosphere, making it the windiest planet.

 

Question 16. Which among the planets does not have a moon?
(a) Mars
(b) Earth
(c) Uranus
(d) Mercury
Answer: (d) Mercury
In simple words: Out of the options given, Mercury does not have any natural satellites. Earth has one moon, Mars has two, and Uranus has many moons.

🎯 Exam Tip: Remember the two planets in our solar system that have no moons: Mercury and Venus.

 

Question 17. The orbital velocity of the satellite depends on its
(a) Shape of orbit
(b) altitude
(c) distance
(d) All of these
Answer: (b) altitude
In simple words: A satellite's speed in orbit depends on how high it is above the Earth. The closer it is, the faster it needs to move to stay in orbit, while satellites further away move slower.

🎯 Exam Tip: Orbital velocity is inversely related to altitude; lower orbits require higher speeds to counteract Earth's gravity effectively.

 

Question 18. Which among the following statements is correct?
(i) Meteors are attracted by the gravitational at force of earth.
(ii) Meteors are travelling in low speeds.
(iii) Most of Meteors are burnt by the heat generated by friction in atmosphere.
(a) (i) & (ii)
(b) (ii) & (iii)
(c) (i) & (iii)
(d) All of these
Answer: (c) (i) & (iii)
In simple words: Meteors are pulled towards Earth by gravity, and as they fall through our atmosphere, the air rubs against them, making them burn up due to intense heat. They actually travel at very high speeds, not low speeds.

🎯 Exam Tip: Understand that meteors burn up due to friction at very high speeds, not low speeds, and that Earth's gravity is always at play.

 

Question 19. The first part of the ISS was launched by
(a) Russian Soyuz
(b) Russian Zarya
(c) Russian Orbital
(d) American Sputnik
Answer: (b) Russian Zarya
In simple words: The first piece of the International Space Station (ISS) that was sent into space was called Zarya, built by Russia. This module provided important initial functions for the station.

🎯 Exam Tip: Recall Zarya as the foundational module of the ISS, signifying the start of international collaboration in space. Sputnik was the first artificial satellite, not related to ISS modules.

 

Question 20. The time is taken by the sun to complete one revolution around the milky way
(a) 250 million years
(b) 200 million years
(c) 225 million years
(d) 240 million years
Answer: (c) 225 million years
In simple words: Our Sun, along with the entire solar system, takes a very long time to travel once around the center of the Milky Way galaxy. This long journey is called a galactic year.

🎯 Exam Tip: Remember that the Sun is also in motion, orbiting the galactic center, and understand the concept of a "galactic year."

 

Question 21. Which of the following statements is correct?
A. There are eight planets in our Solar System.
B. Except Mars, all other planets revolve around the Sun in elliptical orbits.
(a) A only
(b) B only
(c) Both A and
(d) None
Answer: (a) A only
In simple words: There are indeed eight main planets in our solar system. All planets, including Mars, revolve around the Sun in elliptical, not circular, orbits. This is a fundamental law of planetary motion stated by Kepler.

🎯 Exam Tip: Recall the eight planets of our solar system and Kepler's First Law, which states that all planets orbit the Sun in ellipses.

 

II. Fill in the blanks:

 

Question 1. Sun is the star present in the galaxy called
Answer: milky way
In simple words: Our Sun is one of the many stars found in the Milky Way galaxy. The Milky Way is the galaxy where our solar system is located.

🎯 Exam Tip: Remember the name of our home galaxy, the Milky Way.

 

Question 2. The universe began with the start of a massive explosion called
Answer: Big Bang
In simple words: Scientists believe the universe started with a huge burst of energy and matter, which they call the Big Bang. This event led to everything we see today.

🎯 Exam Tip: The Big Bang theory is the leading cosmological model for the universe's origin; know its basic concept.

 

Question 3. Scientists believe that there are galaxies in the observable universe.
Answer: one hundred billion (10\(^{11}\))
In simple words: There are an incredibly large number of galaxies in the part of the universe we can observe. Each of these galaxies contains billions of stars.

🎯 Exam Tip: Appreciate the vast scale of the universe, with billions of galaxies, each containing billions of stars.

 

Question 4. Our closest neighbouring galaxy is
Answer: Andromeda
In simple words: The Andromeda galaxy is the closest large galaxy to our Milky Way. It is a spiral galaxy, and one day, in billions of years, it will collide with our galaxy.

🎯 Exam Tip: Andromeda is an important neighboring galaxy and is visible to the naked eye under dark sky conditions.

 

Question 5. We can see nearly stars with the naked eye in the dark night
Answer: 3000
In simple words: On a very clear, dark night, far away from city lights, a person can see about 3,000 stars without needing a telescope. This number includes stars in both the northern and southern hemispheres.

🎯 Exam Tip: Light pollution significantly reduces the number of stars visible from urban areas.

 

Question 6. The brightness of stars depends on their and from the earth.
Answer: Intensity and distance
In simple words: How bright a star appears to us depends on how much light it actually gives off (its true brightness) and how far away it is from us. A very bright star far away might look dimmer than a less bright star closer to Earth.

🎯 Exam Tip: Distinguish between a star's apparent brightness (how bright it looks from Earth) and its absolute brightness (its true luminosity).

 

Question 7. The Greek name for the Sun is
Answer: Helios
In simple words: In ancient Greek mythology, Helios was the god of the Sun. Many words related to the Sun, like heliocentric, come from this Greek name.

🎯 Exam Tip: Knowing the Greek roots of scientific terms can often help in understanding their meaning.

 

Question 8. The sun is believed to be more than years ago
Answer: 4.6 billion
In simple words: Our Sun is very old, estimated to have formed about 4.6 billion years ago. It is currently in the middle of its life cycle and will continue to shine for billions more years.

🎯 Exam Tip: The age of celestial bodies is determined by various scientific methods, including radiometric dating of meteorites.

 

Question 9. The inner planets are also known as
Answer: Rocky planets
In simple words: The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) are mainly made of rock and metal, so they are called inner or rocky planets. These planets have solid surfaces, unlike the gas giants.

🎯 Exam Tip: Remember the classification of planets into inner (rocky/terrestrial) and outer (gas giants) based on their composition and location.

 

Question 10. The outer planets are also called as
Answer: Gaseous planets
In simple words: The four planets farther from the Sun (Jupiter, Saturn, Uranus, and Neptune) are mostly made of gases like hydrogen and helium, and they don't have solid surfaces. These are also known as gas giants.

🎯 Exam Tip: Outer planets are massive and primarily composed of lighter elements, which gives them their gaseous nature.

 

Question 11. is the brightest heavenly body in our night sky.
Answer: Venus
In simple words: Venus is often the first "star" you see in the evening or the last one in the morning, appearing very bright because it reflects a lot of sunlight and is relatively close to Earth. It's often called the "Morning Star" or "Evening Star".

🎯 Exam Tip: Venus's brightness is due to its proximity and its dense, reflective cloud cover.

 

Question 12. is called as giant planet
Answer: Jupiter
In simple words: Jupiter is the largest planet in our solar system, so big that all the other planets could fit inside it. Its enormous size earns it the title of a "giant planet."

🎯 Exam Tip: Jupiter is the quintessential "gas giant" due to its enormous mass and size, primarily composed of hydrogen and helium.

 

Question 13. is the only moon in the solar system with clouds.
Answer: Titan
In simple words: Titan, a moon of Saturn, is unique because it has a thick atmosphere with clouds, much like a planet. These clouds are made of methane and ethane, not water.

🎯 Exam Tip: Titan is notable for its dense atmosphere and liquid methane lakes, making it a prime target for scientific study.

 

Question 14. Uranus is a giant
Answer: cold gas
In simple words: Uranus is a large planet made mostly of gases like hydrogen, helium, and methane, and it is very cold. It is often referred to as an "ice giant" due to the significant presence of icy materials in its composition.

🎯 Exam Tip: Uranus and Neptune are often categorized as "ice giants" due to their higher abundance of heavier elements compared to Jupiter and Saturn.

 

Question 15. Pluto crosses the orbit of Neptune in every
Answer: 248 years
In simple words: Pluto has an unusual orbit that sometimes takes it inside Neptune's path. This happens every 248 Earth years, but they never collide because their orbits are aligned differently.

🎯 Exam Tip: Pluto's eccentric and inclined orbit is one reason it was reclassified as a dwarf planet.

 

Question 16. The biggest asteroid is
Answer: Ceres
In simple words: Ceres is the largest object in the asteroid belt between Mars and Jupiter. It is so big that it is also classified as a dwarf planet.

🎯 Exam Tip: Ceres is unique as both the largest asteroid and a dwarf planet, highlighting its significant size.

 

Question 17. Satellites move around the planets due to and force.
Answer: gravity, the centripetal
In simple words: Planets keep their moons and artificial satellites in orbit because of their strong gravitational pull. This pull creates a centripetal force that constantly tugs the satellite inward, stopping it from flying off into space.

🎯 Exam Tip: Gravitational force provides the necessary centripetal force for objects to maintain stable orbits around celestial bodies.

 

Question 18. The ratio of the periods of any two planets is equal to the ratio of their semi-major axis from the Sun.
Answer: squares, cubes
In simple words: Kepler's Third Law states that if you take the square of a planet's orbital period and divide it by the cube of its average distance from the Sun, you get the same number for all planets. This law shows a clear mathematical relationship between a planet's orbital time and its distance from the Sun.

🎯 Exam Tip: Accurately stating Kepler's Third Law, including the "square" of the period and "cube" of the semi-major axis, is crucial for full marks.

 

Question 19. The temperature of the star is determined by
Answer: Colour
In simple words: We can tell how hot a star is just by looking at its color. Blue stars are the hottest, yellow stars like our Sun are medium hot, and red stars are the coolest. This is because hotter objects emit light at shorter wavelengths.

🎯 Exam Tip: Remember the correlation: blue/white means hotter, yellow means moderate, and red/orange means cooler for stars.

 

Question 20. is the only moon in the solar system that moves in the opposite direction to the direction in which its planet spins.
Answer: Triton
In simple words: Triton, Neptune's largest moon, has a unique "retrograde" orbit, meaning it moves around Neptune in the opposite direction to the planet's spin. This unusual orbit suggests Triton was likely captured by Neptune's gravity rather than forming with the planet.

🎯 Exam Tip: Triton's retrograde orbit is a key characteristic that sets it apart from most large moons in the solar system.

 

III. Match the following:

 

(I) Match the following:
1. Mercury
2. Saturn
3. Neptune
4. Venus
Answer:
1. Mercury - (c) Rocky planet
2. Saturn - (d) Lightest planet
3. Neptune - (b) Greenish star
4. Venus - (a) Hottest planet
In simple words: This match connects each planet to one of its main features. Mercury is rocky, Saturn is surprisingly light (less dense than water), Neptune has a distinct greenish-blue color, and Venus is the hottest due to its thick atmosphere.

🎯 Exam Tip: Review key characteristics of each planet to accurately match them with their descriptions. Note that 'Greenish star' for Neptune likely refers to its appearance as a green-blue 'star' in the sky, not a literal star.

 

(II) Match the following:
1. Mars
2. Jupiter
3. Earth
4. Uranus
Answer:
1. Mars - (d) Red planet
2. Jupiter - (c) Giant planet
3. Earth - (a) Bluish-green planet
4. Uranus - (b) Cold gas giant
In simple words: This matching exercise helps us remember simple facts about these planets. Mars is red, Jupiter is the largest, Earth looks bluish-green from space because of water and land, and Uranus is a cold planet made of gas.

🎯 Exam Tip: Consolidate knowledge of each planet's defining color, size classification, and composition.

 

(III) Match the following:
1. Ganymede
2. Triton
3. Titan
4. Phobas
Answer:
1. Ganymede - (b) Jupiter
2. Triton - (a) Neptune
3. Titan - (d) Saturn
4. Phobas - (c) Mars
In simple words: This match connects each moon to its parent planet. Ganymede orbits Jupiter, Triton orbits Neptune, Titan orbits Saturn, and Phobos (likely meant instead of Phobas) orbits Mars.

🎯 Exam Tip: Familiarize yourself with the major moons and their respective planets, as they often appear in multiple-choice questions.

 

(IV) Match the following:
1. Jupiter
2. Mercury
3. Venus
4. Saturn
5. Mars
(a) 17.2 hours
(b) 10.7 hours
(c) 87.97 days
(d) 9 hours 55 min
(e) 243 days
(f) 87.97 days
(g) 24 hours 37 min
Answer:
1. Jupiter - (d) 9 hours 55 min
2. Mercury - (c) 87.97 days
3. Venus - (e) 243 days
4. Saturn - (b) 10.7 hours
5. Mars - (g) 24 hours 37 min
In simple words: This match helps us remember how long a day or year is on different planets. Jupiter and Saturn have very short days. Mercury and Venus have very long years, or in the case of Venus, a very long rotational period for its day. Mars's day is just slightly longer than Earth's.

🎯 Exam Tip: Note that "days" can refer to a planet's rotational period, and "hours" can be used for very short rotational periods. Be careful to match the correct unit of time.

 

IV. State whether true or false. If false, correct the statement:

 

Question 1. The basic constituent of the universe is the luminous matter.
Answer: True.
In simple words: The universe is mostly made up of luminous matter, which is the stuff that gives off light, like stars and galaxies. This is what we can see directly.

🎯 Exam Tip: While luminous matter is what we see, remember that dark matter and dark energy make up a much larger portion of the universe's total mass-energy content.

 

Question 2. The important thing about universe is that it is currently shrinking.
Answer: False.
Correct statement: The important thing about the universe is that it is currently expanding.
In simple words: The universe is actually getting bigger all the time, not shrinking. Scientists have observed that galaxies are moving away from each other, showing that space itself is stretching.

🎯 Exam Tip: The expansion of the universe is a cornerstone of modern cosmology, supported by observations like the redshift of distant galaxies.

 

Question 3. All the matter in the universe is made up of hydrogen and helium.
Answer: True.
In simple words: The early universe was almost entirely hydrogen and helium. These two light elements are still the most common elements that make up most of the matter we see in stars and galaxies.

🎯 Exam Tip: While heavier elements exist, they are formed inside stars; the primordial composition of the universe was dominated by hydrogen and helium.

 

Question 4. Stars also appear to be in different colours depending on their composition of Gases.
Answer: False.
In simple words: The color of a star tells us how hot it is, not what gases it is made of. For example, hotter stars look blue, while cooler stars look red. While their composition (mainly hydrogen and helium) is important for their energy production, it is temperature that determines their visual color.

🎯 Exam Tip: Avoid confusing a star's temperature-determined color with its chemical composition; spectral analysis reveals composition.

 

Question 4. Stars also appear to be in different colours depending on their composition of Gases.
Answer: False.
Correct statement: Stars appear in different colours depending on their temperature. This is because the type of gases making up a star is important for its color, but it's really the temperature of those gases that determines the light it gives off.
In simple words: The idea that star colors depend on what they're made of is false; their color actually depends on how hot they are.

🎯 Exam Tip: Remember that a star's color (like blue for hot, red for cool) is a direct indicator of its surface temperature, not just its chemical makeup.

 

Question 5. The planets are revolving around the sun because of the gravitational force of attraction between them.
Answer: True.
The Sun's strong gravitational pull keeps all the planets in their orbits. Without this force, planets would simply move in straight lines into space.
In simple words: Planets orbit the Sun because the Sun's gravity pulls on them, keeping them in place.

🎯 Exam Tip: Gravitational force is key for understanding all orbital motions, from planets around stars to moons around planets.

 

Question 6. The rotation of the sun near the poles takes around 25.4 days.
Answer: False.
Correct statement: The rotation of the Sun near the poles takes around 36 days. The Sun, being a ball of gas, rotates at different speeds at its equator and poles, a phenomenon known as differential rotation.
In simple words: The Sun's poles take longer to spin around than its middle part; it takes about 36 days at the poles.

🎯 Exam Tip: For true/false questions, always know the correct value or fact if the statement is false to score full marks for the correction.

 

Question 7. Mercury is a special planet from the Sun, almost the same size as Earth.
Answer: False.
Correct statement: Venus is a special planet from the Sun, almost the same size as Earth. Mercury is actually the smallest planet in our solar system, much smaller than Earth.
In simple words: The statement is false; Mercury is much smaller than Earth, but Venus is about the same size as Earth.

🎯 Exam Tip: Be precise with planet facts; knowing relative sizes is important for distinguishing them.

 

Question 8. The axis of rotation of the Earth is perpendicular to the plane of its orbit.
Answer: False.
Correct statement: The axis of rotation of the Earth is not perpendicular to the plane of its orbit. It is tilted at approximately 23.5 degrees relative to its orbital plane, which is responsible for seasons.
In simple words: Earth's spin axis is tilted, not straight up and down, compared to its path around the Sun.

🎯 Exam Tip: Understanding the Earth's axial tilt is fundamental to explaining the occurrence of seasons on our planet.

 

Question 9. Pole star is not visible from the southern hemisphere.
Answer: True.
The Pole Star (Polaris) is located almost directly above Earth's North Pole, so it can only be seen from the Northern Hemisphere. Observers in the Southern Hemisphere see different stars and constellations.
In simple words: People in the southern part of the world cannot see the Pole Star because it only shines above the North Pole.

🎯 Exam Tip: The Pole Star is a constant reference point for navigation in the Northern Hemisphere but is not visible south of the equator.

 

Question 10. Comets revolve around the Sun in highly spiral orbits.
Answer: False.
Correct statement: Comets revolve around the Sun in highly elliptical orbits. These oval-shaped paths are very elongated, not spiral, causing comets to spend most of their time far from the Sun.
In simple words: Comets do not orbit the Sun in spirals; they move in very long, oval-shaped paths called ellipses.

🎯 Exam Tip: Differentiate between spiral (like a galaxy) and elliptical (like comet/planet orbits) shapes in astronomy.

 

Question 11. The distance between Saturn and Uranus is about 10 times that between Earth and Mars.
Answer: False.
Correct statement: The distance between Saturn and Uranus is about 20 times that between Earth and Mars. This highlights how much larger the distances become in the outer solar system compared to the inner planets.
In simple words: The gap between Saturn and Uranus is actually about 20 times wider than the gap between Earth and Mars, not 10 times.

🎯 Exam Tip: Be mindful of scale differences when comparing distances in the solar system, especially between inner and outer planets.

V. Assertion and Reason type questions :

 

Question 1. Assertion: Earth appears bluish-green. Reason: This is due to the reflection of light from water and land mass on its surface.
Answer: (a) If both assertion and reason are true and Reason is the correct explanation of Assertion.
In simple words: The Earth looks bluish-green from space because sunlight reflects off its oceans and land, and the reason given correctly explains this appearance.

🎯 Exam Tip: For assertion-reason questions, first check if both statements are individually true, then check if the reason correctly explains the assertion.

 

Question 2. Assertion: Sun is a medium-sized star. Reason: Sun is situated at the centre of the solar system.
Answer: (b) Both assertion and reason are true but the reason is not the correct explanation of the assertion
In simple words: Both statements are true: the Sun is a medium star and it is at the center of our solar system. However, the Sun's size is not explained by its position in the solar system.

🎯 Exam Tip: A reason might be factually correct but still not be the *explanation* for the assertion; always check for direct causal links.

 

Question 3. Assertion : The outer planets are called as Terrestrial Planets. Reason: Outer planets are made up of hydrogen, helium and other gases in huge amounts and have very dense atmospheres.
Answer: (d) Assertion is false but reason is true
Assertion : The outer planets are called as Gas giants. These planets are primarily composed of gases, which is accurately described in the reason. Terrestrial planets are rocky, like Earth.
In simple words: The outer planets are called "Gas giants," not "Terrestrial Planets," so the assertion is false. But it is true that they are made of a lot of hydrogen and helium gases with thick atmospheres.

🎯 Exam Tip: Remember the distinction between "terrestrial" (rocky, inner) and "gas giant" (gaseous, outer) planets.

 

Question 4. Assertion : The period of revolution of a satellite around a planet is directly proportional to the radius of the orbit of the satellite. Reason: Artificial satellites do not follow Kepler's Laws of motion.
Answer: (c) Assertion is true but reason is false
Reason: Artificial satellites follow Kepler's laws of motion. Kepler's laws apply to any body orbiting another body under gravity, including artificial satellites. The period of revolution is proportional to the radius raised to the power of 3/2, which is a direct relationship.
In simple words: The first part is true: how long a satellite takes to go around a planet depends on how big its orbit is. But the second part is false: man-made satellites *do* follow Kepler's laws of motion.

🎯 Exam Tip: Kepler's laws are universal for orbital motion under gravity, applying to both natural celestial bodies and artificial satellites.

 

Question 5. Assertion : At the pole, the value of acceleration due to gravity (g) is greater than equator. Reason: Earth rotates on its axis in addition to revolving around the sun.
Answer: (a) Both Assertion and Reason are true and Reason is the correct explanation of Assertion
In simple words: Both statements are true. The pull of gravity is stronger at the poles than at the equator because of Earth's rotation and its slightly flattened shape. However, the reason provided only states a general fact about Earth's movement and doesn't directly explain *why* gravity is stronger at the poles due to these rotational effects.

🎯 Exam Tip: The reason why gravity is stronger at the poles is due to Earth's rotation causing a centrifugal force effect at the equator, and the Earth's slightly oblate spheroid shape, not just the fact that it rotates.

VI. Very Short Answer questions :

 

Question 1. What is the universe?
Answer: The universe is everything that exists in space. It includes all matter, energy, planets, stars, galaxies, and the space between them. It is vast and constantly expanding.
In simple words: The universe is everything around us and beyond, including stars, planets, and space itself.

🎯 Exam Tip: When defining broad terms like "universe," ensure your definition is comprehensive yet simple, covering all its main components.

 

Question 2. What is a galaxy?
Answer: A galaxy is a huge group of gas, dust, billions of stars, and their solar systems, all held together by gravity. Our solar system is part of the Milky Way galaxy.
In simple words: A galaxy is a giant group of stars, gas, and dust held together by gravity.

🎯 Exam Tip: Mentioning the role of gravity in holding a galaxy together is a key point in its definition.

 

Question 3. How are galaxies classified?
Answer: Galaxies are classified based on how they look. The main types are:
• Spiral, which have arms that curve outwards like a pinwheel.
• Elliptical, which are oval-shaped.
• Irregular, which have no clear shape.
Astronomers use these shapes to understand how galaxies form and evolve.
In simple words: Galaxies are put into groups based on their shapes, like spiral, oval (elliptical), or no clear shape (irregular).

🎯 Exam Tip: Knowing the three primary classifications (spiral, elliptical, irregular) is essential for describing galaxies.

 

Question 4. Why is our galaxy called Milky Way?
Answer: Our galaxy is called the Milky Way because it looks like a faint, milky band of light across the night sky when seen from Earth. This appearance is due to the combined light from countless stars too far away to be seen individually.
In simple words: Our galaxy got its name, Milky Way, because it looks like a milky strip of light across the night sky.

🎯 Exam Tip: Connect the name "Milky Way" directly to its visual appearance in the night sky from our perspective.

 

Question 5. What is an orbit?
Answer: An orbit is the specific curved path that a planet follows as it moves around the Sun. Similarly, moons orbit planets, and satellites orbit Earth, all following a definite path due to gravity.
In simple words: An orbit is the curved path a planet or satellite takes around another, bigger object.

🎯 Exam Tip: Emphasize that an orbit is a "definite curved path" to differentiate it from random movement.

 

Question 6. Define Period of rotation.
Answer: The period of rotation is the time it takes for a planet or any celestial body to spin once completely on its own axis. For Earth, this period is approximately 24 hours.
In simple words: The period of rotation is how long it takes for something, like a planet, to spin around one full time.

🎯 Exam Tip: Clearly state that rotation is about spinning on an axis, while revolution is about orbiting another body.

 

Question 7. Give a reason for changes of seasons on Earth.
Answer: The changes in seasons on Earth happen because:
• Earth's axis is not straight up and down compared to its orbit. It is tilted.
• This tilt is the main reason for different seasons on Earth, as it changes how much sunlight different parts of the planet receive throughout the year.
Without this tilt, we wouldn't have distinct seasons.
In simple words: Seasons change because the Earth's axis is tilted, which makes different parts of the Earth get more direct sunlight at different times of the year.

🎯 Exam Tip: The axial tilt of the Earth is the most crucial factor for seasons; always highlight this point.

 

Question 8. Mention the factors responsible for motion of satellite.
Answer: The motion of a satellite is determined by two main factors:
• Gravity, which constantly pulls the satellite towards the central body (like Earth).
• The centripetal force, which is the force that keeps the satellite moving in a curved path around the central body. These forces work together to maintain a stable orbit.
In simple words: Satellites move because of gravity pulling them towards the Earth and a force that keeps them moving in a circle.

🎯 Exam Tip: Understanding that gravity *provides* the centripetal force for orbital motion is key to explaining satellite movement.

 

Question 9. What is a geostationary orbit?
Answer: A geostationary orbit is a special type of orbit where a satellite stays fixed in position above one point on Earth's surface. This happens because the satellite moves around the Earth at the same speed and in the same direction as the Earth spins, taking exactly 24 hours to complete one revolution. This type of orbit is very useful for communication satellites.
In simple words: A geostationary orbit is when a satellite circles the Earth at the same speed as Earth spins, making it look like it's always in the same spot in the sky.

🎯 Exam Tip: The key characteristic of a geostationary orbit is that the satellite appears motionless from the ground, making it ideal for continuous communication.

 

Question 10. How is eye-tracking technology helpful?
Answer: Eye-tracking technology is very helpful, especially for people with disabilities. It allows them to control computers or communicate using just their eye movements. This can help individuals with limited body movement or speech to interact with the world around them, improving their quality of life.
In simple words: Eye-tracking technology helps people, especially those with disabilities, use their eyes to control devices or talk, when they can't move much or speak.

🎯 Exam Tip: Focus on the practical applications and beneficiaries of eye-tracking technology, particularly for assistive communication.

 

Question 11. What is a cosmic year?
Answer: A cosmic year is the time it takes for our Sun to complete one full revolution around the center of the Milky Way galaxy. The Sun travels at an incredible speed of about 250 km per second (900,000 km/h), and it takes approximately 225 million years to complete this journey. This long period highlights the vastness of our galaxy.
In simple words: A cosmic year is the super-long time it takes for our Sun to go once around the middle of our galaxy, which is about 225 million years.

🎯 Exam Tip: Remember the colossal time scale of a cosmic year, reflecting the immense size of the galaxy.

 

Question 12. Why are we able to see the Moon even though it is not a luminous body?
Answer: We can see the Moon even though it doesn't make its own light because it reflects light from the Sun. The Moon acts like a giant mirror, bouncing sunlight towards Earth. This reflected sunlight is what makes the Moon appear bright in our night sky.
In simple words: We see the Moon because it bounces sunlight off its surface, not because it makes its own light.

🎯 Exam Tip: The concept of reflected light is crucial for understanding why non-luminous objects, like planets and moons, are visible to us.

VII. Answer briefly :

 

Question 1. Write notes on the Milky Way.
Answer:
1. The Milky Way galaxy has a spiral shape, resembling a giant pinwheel in space.
2. It is made up of about 100 billion stars, along with gas and dust.
3. Its total width is approximately 1,00,000 light-years across, showcasing its immense size.
4. Our own solar system is located about 25,000 light-years away from the center of this galaxy. The center is a region of very dense stars and a supermassive black hole.
In simple words: The Milky Way is our galaxy; it's spiral-shaped, has billions of stars, is very wide (100,000 light-years), and our Sun is 25,000 light-years from its middle.

🎯 Exam Tip: Include key characteristics like shape, number of stars, and the size (diameter and Sun's location) for a comprehensive answer.

 

Question 2. How does a planet differ from a star?
Answer: A planet and a star have several key differences:

Stars are the powerhouses of the universe, while planets are their orbiting companions.
In simple words: Stars create their own light and heat, while planets only reflect light from stars. Stars are big and hot, but planets are smaller and cooler.

🎯 Exam Tip: The ability to produce light (luminous vs. non-luminous) is the fundamental difference between a star and a planet.

 

Question 3. Briefly explain (i) brightness (ii) colour of star.
Answer:
(i) Brightness of star: The brightness of a star depends on two things: how much light it actually gives off (its true luminosity) and its distance from Earth. A very luminous star far away might appear dimmer than a less luminous star that is much closer to us. This is why some stars look brighter than others in the night sky.
(ii) Colour of star: The color of a star is mainly determined by its surface temperature. For example:
1. Hot stars usually appear white or blue.
2. Cooler stars appear orange or red. This is similar to how a metal rod glows red when it's warm and then turns white or blue when it's very hot.
In simple words: A star's brightness depends on how much light it makes and how far away it is. Its color tells us how hot it is: blue or white means hot, and orange or red means cooler.

🎯 Exam Tip: For brightness, remember the dual factors of intrinsic luminosity and distance. For color, link it directly to temperature, using examples to illustrate the concept.

 

Question 4. What is a constellation?
Answer:
1. A constellation is a group of stars that form an imaginary shape or meaningful pattern in the night sky. These patterns were created by ancient cultures to help them tell stories and track seasons.
2. Often, these star patterns are thought to look like animals, mythological people, or everyday objects. For instance, the Big Dipper is part of the Ursa Major constellation.
In simple words: A constellation is a group of stars that people imagine form a specific shape, like an animal or a person, in the night sky.

🎯 Exam Tip: Emphasize that constellations are "imaginary" patterns used for navigation and storytelling, not physically connected groups of stars.

 

Question 5. How is life on earth supported?
Answer:
1. Life on Earth is mainly supported by the Sun, which provides the necessary energy in the form of sunlight. This energy is essential for photosynthesis in plants, forming the base of most food chains.
2. Solar energy has continuously helped maintain and sustain life on Earth since its beginning. Other factors like the presence of water, a suitable atmosphere, and Earth's distance from the Sun also play crucial roles in supporting life.
In simple words: The Sun's energy supports life on Earth by providing light for plants. It has done this since the Earth began, making life possible with other things like water and air.

🎯 Exam Tip: When discussing life support, remember to mention the Sun's energy, water, atmosphere, and Earth's optimal distance from the Sun.

 

Question 6. Write short notes on Asteroids.
Answer:
1. Asteroids are small, rocky objects that were left over from when the planets first formed in our solar system. There are half a million or more of these pieces of rock.
2. Asteroids primarily revolve around the Sun, mostly found in a region between Mars and Jupiter called the asteroid belt. They follow their own orbits.
3. Most asteroids can only be seen with a large telescope because they are quite small.
4. The biggest known asteroid is Ceres, which is also classified as a dwarf planet. Asteroids give us clues about the early solar system.
In simple words: Asteroids are leftover rocks from when planets were made. They orbit the Sun, mostly in a belt between Mars and Jupiter. You need a big telescope to see them, and Ceres is the largest one.

🎯 Exam Tip: Key points for asteroids include their origin (leftovers), location (asteroid belt), size (usually small, seen with telescope), and the largest example (Ceres).

 

Question 7. Differentiate Meteors and Meteorites?
Answer: Meteors and meteorites are related but refer to different stages of a space rock's journey:

The key difference is whether the space rock survives its fiery descent through the atmosphere.
In simple words: A meteor is a space rock that burns up in the sky, making a "shooting star." A meteorite is a space rock that is big enough to pass through the atmosphere and land on Earth.

🎯 Exam Tip: The distinction lies in whether the celestial body reaches Earth's surface: a meteor burns up, a meteorite lands.

 

Question 8. Write down the formula of orbital velocity.
Answer: The formula for orbital velocity (v) of a satellite orbiting a celestial body is:
\[ v = \sqrt{\frac{GM}{(R+h)}} \]
Where:
\( v \) = Orbital velocity.
\( G \) = Gravitational constant (6.673 × \( 10^{-11} \) Nm\(^2\)kg\(^{-2}\))
\( M \) = Mass of the Earth (or central body)
\( R \) = Radius of the Earth (or central body)
\( h \) = Height of the satellite from the surface of the Earth. This formula helps us understand how fast an object needs to move to stay in orbit.
In simple words: The formula for orbital velocity uses the gravitational constant, the mass of the planet, its radius, and the height of the satellite. It calculates how fast a satellite needs to move to stay in its path.

🎯 Exam Tip: Clearly define each variable in the formula to show a complete understanding of orbital velocity calculations.

 

Question 9. Write short notes on Halley's comet.
Answer:
1. Halley's Comet is a famous periodic comet, meaning it returns to Earth's vicinity at regular intervals.
2. It appears in the sky approximately every 76 years, making it visible to humans roughly once in a lifetime.
3. It was last seen in 1986 and is expected to be visible again in 2062. Comets like Halley's are icy bodies that release gas and dust as they approach the Sun, forming a visible tail.
In simple words: Halley's Comet is a well-known comet that comes near Earth about every 76 years. It was last seen in 1986 and will return in 2062.

🎯 Exam Tip: Remember Halley's Comet for its regular appearance cycle and its status as a periodic comet.

VIII. Answer In detail:

 

Question 1. Explain the salient features of the Big Bang Theory.
Answer: According to the Big Bang Theory, the universe began from a very hot and dense state and has been expanding ever since. Here are its main features:
1. The universe started as an extremely hot and dense point of matter and energy.
2. Approximately 13.7 billion years ago, a massive explosion, known as the Big Bang, occurred, marking the beginning of the universe.
3. All the matter and energy from this explosion were ejected outwards in every direction, leading to the formation of galaxies.
4. These galaxies are made up of stars, which are held together by the force of gravity.
5. As the universe continued to expand, stars exploded (supernovae) and formed planets, which were then scattered throughout the universe. This theory is widely accepted as the leading scientific explanation for the origin and evolution of the universe.
In simple words: The Big Bang Theory says our universe started from a very hot, tiny point and then exploded about 13.7 billion years ago. This explosion sent matter everywhere, which eventually formed galaxies, stars, and planets, and the universe is still growing.

🎯 Exam Tip: Key points for the Big Bang Theory are the initial hot/dense state, the timing (13.7 billion years ago), the expansion, and the formation of galaxies/stars.

 

Question 2. How is Sun formed according to Big Bang theory?
Answer: According to the Big Bang theory, the Sun formed as part of a larger process following the universe's initial expansion:
1. After the Big Bang, hydrogen gas in the early universe began to cool and condense, forming vast clouds.
2. Some of this hydrogen gas remained free-floating, while other parts started to gather in particular regions. These concentrated areas eventually led to the formation of stars and solar systems, including our Sun.
3. The areas where hydrogen gas gathered more densely became "paved ways" for the formation of celestial bodies.
4. Our Sun and its solar system specifically emerged from a slowly rotating cloud made of hydrogen and helium molecules. This cloud began to spin faster over time.
5. Due to gravity, these clouds underwent compression, pulling the gas and dust closer together.
6. The rapid and excessive spinning of this cloud caused it to flatten into a giant disc, with the central part eventually forming the Sun. This process of gravitational collapse and rotation is a common way stars are born.
In simple words: After the Big Bang, hydrogen gas gathered into big clouds. These clouds spun faster and gravity pulled them together, making them flat discs. The middle of one of these discs became our Sun.

🎯 Exam Tip: Focus on the sequence: gas clouds, gravitational collapse, spinning, flattening into a disc, and central concentration for star formation.

 

Question 3. Give a comparative account of satellites and rings of planets.
Answer: Satellites and planetary rings are both celestial objects that orbit planets, but they have distinct differences:

Satellites are generally larger, individual bodies, while rings are made of countless small particles. Both are important components of a planet's system, but they differ significantly in structure and composition.
In simple words: Satellites (moons) are single, larger objects orbiting a planet, while rings are made of many tiny pieces of rock and ice that spread out in flat circles around a planet. Some planets have both, and some have neither.

🎯 Exam Tip: Highlight that satellites are discrete bodies, while rings are collections of small particles, and note which planets have each feature.

IX. Numerical problems :

 

Question 1. The Earth's distance from the Sun is 149.6 x \( 10^6 \) km and the period of Earth's revolution is 1 year. Calculate the ratio of the squares of the periods to cubes of the major axis.
Answer:
Semi-major axis from Sun, \( R = 149.6 \times 10^6 \) km.
Period of Earth, \( T = 1 \) year.
Now, we calculate the ratio \( \frac{T^2}{R^3} \):
\( \frac{T^2}{R^3} = \frac{(1)^2}{(149.6 \times 10^6)^3} \)
\( \implies \frac{T^2}{R^3} = \frac{1}{(149.6)^3 \times (10^6)^3} \)
\( \implies \frac{T^2}{R^3} = \frac{1}{3348071.1 \times 10^{18}} \)
\( \implies \frac{T^2}{R^3} \approx 2.98 \times 10^{-25} \) year\(^2\) / km\(^3\). This ratio is a constant for all planets orbiting the Sun, according to Kepler's Third Law.
In simple words: We are finding a special ratio by taking the square of Earth's orbital time and dividing it by the cube of its distance from the Sun. This gives us a tiny number, which is a constant for objects orbiting the Sun.

🎯 Exam Tip: Remember Kepler's Third Law states that the ratio of the square of the orbital period to the cube of the semi-major axis is a constant for all objects orbiting the same central body.

 

Question 2. The mean distance of Earth from the Sun is 149.6 × \( 10^6 \) km and the mean distance of Mercury from the sun is 57.9× \( 10^6 \) km. The period of Earth's revolution is 1 year. What is the period of Mercury's revolution?
Answer:
Given:
1. Mean distance of Earth from Sun, \( r_1 = 149.6 \times 10^6 \) km.
2. Mean distance of Mercury from Sun, \( r_2 = 57.9 \times 10^6 \) km.
3. The period of Earth's revolution, \( T_1 = 1 \) year.
4. The period of Mercury's revolution, \( T_2 = ? \)

According to Kepler's Third Law:
\( \frac{T_1^2}{r_1^3} = \frac{T_2^2}{r_2^3} \)
\( \implies T_2^2 = T_1^2 \times \frac{r_2^3}{r_1^3} \)
\( \implies T_2^2 = (1)^2 \times \frac{(57.9 \times 10^6)^3}{(149.6 \times 10^6)^3} \)
\( \implies T_2^2 = 1 \times \frac{(57.9)^3 \times (10^6)^3}{(149.6)^3 \times (10^6)^3} \)
\( \implies T_2^2 = \frac{57.9^3}{149.6^3} \)
\( \implies T_2^2 = \frac{193853.319}{3348071.1} \)
\( \implies T_2^2 \approx 0.057889 \) year\(^2\)
Now, we find \( T_2 \):
\( T_2 = \sqrt{0.057889} \)
\( T_2 \approx 0.2406 \) year.

To convert this to days (since 1 year is approx 365.25 days):
\( T_2 = 0.2406 \times 365.25 \)
\( T_2 \approx 87.89 \) days.
Therefore, the period of Mercury's revolution is approximately 0.2407 years or about 87.85 days. This is much shorter than Earth's year, as Mercury is closer to the Sun.
In simple words: Using Kepler's Third Law, we found that Mercury takes about 0.24 years (or almost 88 days) to orbit the Sun once. This is much faster than Earth's one-year orbit because Mercury is closer to the Sun.

🎯 Exam Tip: Pay close attention to unit consistency (km, years) and ensure calculations are done carefully, especially with large numbers and powers.

 

Question 3. Calculate the speed and period of revolution of satellite orbiting at a height 700 km above the Earth's surface.
Answer:
Given:
1. Height of the satellite, \( h = 700 \) km = \( 700 \times 10^3 \) m.
2. Gravitational constant, \( G = 6.673 \times 10^{-11} \) Nm\(^2\)kg\(^{-2}\).
3. Mass of the Earth, \( M = 5.972 \times 10^{24} \) kg.
4. Radius of the Earth, \( R = 6371 \) km = \( 6371 \times 10^3 \) m.

First, calculate \( (R+h) \):
\( R+h = (6371 + 700) \) km = \( 7071 \) km = \( 7071 \times 10^3 \) m.

**1. Calculate Speed of Satellite (Orbital Velocity), \( v \):**
The formula for orbital velocity is:
\[ v = \sqrt{\frac{GM}{(R+h)}} \]
Substitute the values:
\( v = \sqrt{\frac{(6.673 \times 10^{-11}) \times (5.972 \times 10^{24})}{(7071 \times 10^3)}} \)
\( \implies v = \sqrt{\frac{39.85 \times 10^{13}}{7.071 \times 10^6}} \)
\( \implies v = \sqrt{5.6357 \times 10^7} \)
\( \implies v \approx 7507 \) m/s.
This is a very high speed, typical for satellites in low Earth orbit.

**2. Calculate Period of Revolution, \( T \):**
The formula for the period of revolution is:
\( T = \frac{2\pi(R+h)}{v} \)
Substitute the values:
\( T = \frac{2 \times 3.14 \times (7071 \times 10^3)}{7507} \)
\( \implies T = \frac{44415492}{7507} \)
\( \implies T \approx 5916.6 \) seconds.

To convert seconds to minutes:
\( T \approx \frac{5916.6}{60} \)
\( T \approx 98.6 \) minutes.
So, the period of revolution is approximately 5915 seconds (or 98 minutes 35 seconds). This means the satellite completes one orbit around Earth in just under 100 minutes.
In simple words: We first find the total distance from Earth's center to the satellite. Then, we use special formulas to calculate the satellite's speed, which is about 7507 meters per second. Next, we find how long it takes to go around Earth once, which is about 5915 seconds or roughly 98 minutes and 35 seconds.

🎯 Exam Tip: Ensure all units are consistent (meters, kilograms, seconds) before calculation. Pay attention to powers of 10 and scientific notation during intermediate steps.

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