CBSE Class 7 Science Advanced Physics Our Universe Short Answer Questions Set 01

Short Answer Questions

Question 1. What are neap tides? Explain the formation of neap tides.
Answer:
Neap tides are low tides that occur when the gravitational pulls of the moon and the sun on the earth are perpendicular to each other.
They occur during the first and the last quarter phases of the moon.
The pulls of the sun and the moon act at right angles, so the net pull on the water is small. The tide is therefore weaker than normal.

In simple words: When the sun and moon pull from directions at 90°, they hardly add up, giving weak tides.

Teacher's Note:
1. Draw the sun, earth and moon at right angles.
2. Compare with spring tides, where the three are in a line.

Question 2. Mention the good effects of tides.
Answer:
1. Tidal water makes rivers navigable and flushes out wastes from rivers, which keeps river water clean.
2. Fishes such as Salmon and Hilsa climb up rivers to lay eggs with the help of tidal water, and fishermen catch them in good numbers.
3. Salt is made from sea water that rises on land during high tides.
4. The energy of tidal water is used to generate electricity.
In simple words: Tides help navigation, fishing, salt making and electricity generation.

Teacher's Note:
1. List at least four effects.
2. Name the fishes for full marks.

Question 3. What happens to a ship that moves from a high density region to a low density region of the sea? Explain.
Answer:
The ship sinks deeper in the water, and it may even sink.
A floating body is immersed more in a liquid of lower density. In a low density region the water gives less upthrust for the same immersion, so the ship has to sink deeper to float.
That is why ships have markings on the hull showing the safe depth of immersion at a given place.
In simple words: In lighter water the ship sits lower, and if loaded too much it can sink.

Teacher's Note:
1. Link density with the depth of immersion.
2. The marks on the hull are the safety limits.

Question 4. Write short notes on India’s space program.
Answer:
India sent its first artificial satellite, ‘Arya Bhatta’, into space with the cooperation of Russia.
Rocket launching stations were set up at Thumba in Kerala and Sriharikota in Andhra Pradesh.
Other satellites launched by India include Bhaskara I and II, Rohini, I.R.S, Apple and Insat IA, IB, IC and ID, used for telecommunication, radio and TV transmission, weather forecasting and agricultural research.
On 22 October 2008, India’s PSLV-C11 launched Chandrayan-1 from Sriharikota, the first Indian spacecraft sent to the moon.
In simple words: India builds its own satellites and rockets and has even sent a spacecraft to the moon.

Teacher's Note:
1. Mention the first satellite and the launch sites.
2. Add Chandrayan-1 to show progress.

Question 5. (a) What are the destructions caused by cyclones?
(b) What happens when high speed winds blow over the roofs of the buildings that are weak?
Answer:
(a) The low pressure at the eye can lift the sea surface by 3 to 12 m, flooding low-lying coastal areas. Continuous heavy rain and high speed winds cause severe loss of life and property.
(b) Fast wind above a roof reduces the air pressure there. The higher pressure inside the building pushes the roof up, and the weak roof is lifted and blown away.
In simple words: A cyclone floods coasts and wrecks property, and strong wind lowers pressure over roofs so that they lift off.

Teacher's Note:
1. Explain (b) using pressure difference.
2. Fast air means low pressure.

Question 6. A body is placed over the surfaces of two liquids of densities d₁ and d₂ (d₁ > d₂) separately. If the density of the object is less than both the liquids, in which liquid the body immerses more?
Answer:
The body immerses more in the liquid of smaller density, d₂.
The part immersed is the fraction (density of body ÷ density of liquid), which is larger for the less dense liquid.
In simple words: The lighter liquid supports less, so the body sinks in more.

Teacher's Note:
1. Denser liquid, less immersion.
2. Use the fraction immersed formula.

Question 7. What are the weather conditions that lead to the formation of a cyclone?
Answer:
When water vapour in the atmosphere condenses into raindrops, it releases heat and warms the air around.
This warm air moves up and the pressure drops, and cool air rushes in towards the centre.
This may end with a very low pressure system with very high speed winds revolving around it, which is a cyclone.
Wind speed, wind direction, temperature and humidity contribute to its development.
In simple words: Warm humid air rising rapidly and cool air rushing in make a spinning low-pressure storm.

Teacher's Note:
1. Follow the order of events.
2. Name the four factors.

Question 8. Write a few uses of a barometer.
Answer:
1. It is used to find the atmospheric pressure at a place.
2. It can be used as an altimeter to know the height of a place above sea level.
3. By watching the changes in atmospheric pressure, rainfall and the arrival of storms can be forecast.
In simple words: A barometer tells the pressure, the height of a place and warns of storms.

Teacher's Note:
1. Three uses are enough.
2. Altimeter is a good keyword.

Teacher's Note:
1. Use a table.
2. Write the units.

Question 10. Explain why do we feel that our weight is decreased in a lift descending with uniform acceleration.
Answer:
When the lift descends with an acceleration, we and the floor move down together, so the floor presses less on us.
The weight we feel is the reaction of the floor, so it appears to be less. In a freely falling lift it would be zero.
So we feel our weight to be decreased.
In simple words: The floor falls away from under us, so it presses less and we feel lighter.

Teacher's Note:
1. The felt weight is the push of the floor.
2. Free fall makes it zero.

Question 11. What are the functions of artificial satellites?
Answer:
Artificial satellites are put in orbits to carry out functions such as communication, radio and TV transmission and weather reporting.
They are also used for navigation, for locating minerals and water, and for studying the earth and space.
In simple words: Satellites carry signals, watch the weather and gather information from space.

Teacher's Note:
1. Give at least three functions.
2. They revolve around the earth.

Question 12. What are the bad effects of tides?
Answer:
1. Tidal water carries sediments and deposits them in rivers, which makes the river water undrinkable and navigation difficult.
2. Tides sometimes make fishing difficult for fishermen.
3. Rivers may overflow due to tidal water, damaging crops and the homes of coastal people.
In simple words: Tides can spoil river water, hinder fishing and cause floods on the coast.

Teacher's Note:
1. Name three effects.
2. Compare with the good effects.

Question 13. Tides are formed mainly due to the gravitational force of attraction between water, the sun and the moon. Why are tides not created by the effect of other planets?
Answer:
The gravitational force is proportional to the product of masses and inversely proportional to the square of the distance.
The moon is very near the earth and the sun is very massive, but the other planets are far away, so their pull on the earth’s water is very small.
Hence other planets cannot produce tides.
In simple words: Other planets are too far away to pull the sea noticeably.

Teacher's Note:
1. Use the inverse square law.
2. Distance is the key factor.

Question 14. In a barometer the height of the mercury column suddenly changes from 76 cm to 60 cm at a particular place. What does it indicate?
Answer:
A sudden fall in the mercury column means a sudden fall in the air pressure.
It indicates the approach of a storm.
In simple words: A sharp fall in the column warns of stormy weather.

Teacher's Note:
1. Low pressure goes with storms.
2. Mention the drop of 16 cm.

Question 15. Is energy required to be supplied for an artificial satellite to revolve in a given orbit?
Answer:
No. The satellite revolves because of the gravitational pull of the earth, which keeps it in orbit. Energy is needed only to launch it into the orbit with the right speed.
In simple words: Gravity keeps the satellite going round without extra fuel.

Teacher's Note:
1. Energy is needed for launching only.
2. The earth’s pull provides the circular motion.

Question 16. The atmosphere exerts pressure on us. Why do we not feel it?
Answer:
Our body fluids, such as blood, exert a pressure slightly more than the atmospheric pressure from inside.
The inside and outside pressures nearly balance, so we do not feel the atmospheric pressure.
In simple words: The pressure inside our body balances the pressure of the air outside.

Teacher's Note:
1. Balanced pressures are not felt.
2. Compare with the crushed container from the notes.

Question 17. Akhilesh took sea water and river water in two containers. He made a body float in the river water and then in the sea water. What happens to the immersed part of the body when it is immersed in the river water and in the sea water?
Answer:
Sea water is denser than river water.
The immersed part of the floating body is less in the sea water than in the river water, so the body rises a little when it is moved to sea water.
In simple words: The denser sea water pushes the body up higher.

Teacher's Note:
1. Denser liquid gives less immersion.
2. Sea water is salty and denser.

Question 18. W_P and W_e be the weight of a body at the pole and at the equator, respectively. If the earth stops rotating, then what happens to the value of W_P and W_e? Are the values of W_e and W_P equal?
Answer:
g is greater at the poles than at the equator because the poles are nearer the centre of the earth.
Because of rotation, the outward (centrifugal) effect is greatest at the equator. If the earth stops rotating, W_e will increase and W_P will remain unchanged.
Still W_e and W_P will not be exactly equal, since the equator is farther from the centre of the earth.
In simple words: Without rotation the equator weight rises, but the pole is still a bit heavier.

Teacher's Note:
1. The poles are closer to the earth’s centre.
2. Rotation reduces the weight at the equator.

Question 19. An object of mass 2 kg is placed on the surface of the earth and another object of mass 6 kg is placed on the surface of the moon. Determine the ratio of their weights.
Answer:
g_e = 6 g_m.
W₁ = 2 g_e = 12 g_m and W₂ = 6 g_m.
W₁ : W₂ = 12 : 6 = 2 : 1.
In simple words: The 2 kg object on the earth weighs twice as much as the 6 kg object on the moon.

Teacher's Note:
1. Write both weights in terms of g_m.
2. Cancel g_m.

Question 20. Aparna drops a body of mass ‘m’ from the top of a tower and she found that it reaches the ground in 4 s. Another body of mass 10 m is dropped from the top of another tower and it reaches the ground in 8 s. Using this information she found the ratio of the height of the towers. What is the ratio of their heights?
Answer:
The distance fallen is proportional to the square of the time, s ∝ t², and it does not depend on the mass.
h₁ : h₂ = 4² : 8² = 16 : 64 = 1 : 4.
In simple words: Twice the time means four times the height.

Teacher's Note:
1. Ignore the masses.
2. Square the times.

Question 21. A student placed two identical spheres A and B as shown in the figure. Then if another ball C identical to A and B is placed in between the balls A and B, determine the ratio of force of attraction between A and B in the two cases.

Answer:
Let r be the radius and m the mass of each ball.
First case: the centres of A and B are 2r apart, so F₁ = G m² ÷ (2r)² = G m² ÷ 4r².
Second case: with C between them, the centres of A and B are 4r apart, so F₂ = G m² ÷ (4r)² = G m² ÷ 16r².
F₁ : F₂ = 16 : 4 = 4 : 1.
In simple words: The balls get twice as far apart, so the force becomes one-fourth.

Teacher's Note:
1. Count the distance between centres.
2. Twice the distance gives one-fourth of the force.

Question 22. If the density of the earth is doubled keeping its radius constant, then the new value of force of attraction on an object on the surface of the earth is how many times that in the previous case?
Answer:
Mass = density × volume. With the same radius, the volume is the same, so the mass of the earth is doubled.
Force of attraction is proportional to the mass of the earth, so the new force is 2 times the previous one.
In simple words: Doubling the density doubles the mass and so doubles the pull.

Teacher's Note:
1. Radius is unchanged, so only the mass matters.
2. F ∝ M.

Teacher's Note:
1. Put every g over the same Gm/R².
2. Radius changes act as the square.

Question 24. Sindhu wanted to know the variation in the gravitational force of attraction with height. So, she took a body of mass ‘m’ from the surface of the earth to a height equal to the radius of the earth. She then determined the change in gravitational force of attraction. What is her answer?
Answer:
On the surface, F₁ = GMm/R².
At a height R above the surface, the distance from the centre is 2R, so F₂ = GMm/(2R)² = F₁/4.
Change = F₁ − F₁/4 = ¾ F₁. The force decreases by three-fourths of its original value.
In simple words: At twice the distance from the centre, the force is one-fourth, so it drops by three-quarters.

Teacher's Note:
1. Distance is measured from the earth’s centre.
2. The force falls to one-fourth, so the change is three-fourths.

Question 25. If the mass and radius of earth are doubled, then what is the weight of an object on the surface of the earth whose mass is 10 kg on the surface of the moon?
Answer:
Mass is the same everywhere, so the object has a mass of 10 kg.
g = GM/R². If M and R are both doubled, g becomes 2 ÷ 4 = ½ of its old value.
Old weight = 10 kgwt, so new weight = 5 kgwt (about 49 N).
In simple words: The new g is half, so a 10 kg object weighs 5 kgwt.

Teacher's Note:
1. Mass does not change.
2. Doubling R divides g by 4 and doubling M multiplies it by 2.