NCERT Solutions Class 7 Science Curiosity Chapter 08 Measurement of Time and Motion

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Question 1. Calculate the speed of a car that travels 150 metres in 10 seconds. Express your answer in km/h.
Answer: First, let us list the values given to us:
Distance = \( 150 \text{ m} \)
Time = \( 10 \text{ s} \)
We can calculate the speed using this formula:
Speed = \( \frac{\text{Distance}}{\text{Time}} \)
Speed = \( \frac{150 \text{ m}}{10 \text{ s}} = 15 \text{ m/s} \)
Now, let us convert this speed into kilometers per hour (\( \text{km/h} \)) by multiplying it by \( \frac{18}{5} \):
Speed = \( 15 \times \frac{18}{5} = 3 \times 18 = 54 \text{ km/h} \)
Thus, the speed of the car is \( 15 \text{ m/s} \) or \( 54 \text{ km/h} \).
In simple words: To find the speed, divide the distance by the time. Then, multiply the result by 18/5 to change it into kilometers per hour.

Exam Tip: Always state the formula clearly before plugging in numbers. When converting from meters per second to kilometers per hour, using the shortcut factor of \( \frac{18}{5} \) saves valuable time.

Question 2. A runner completes 400 metres in 50 seconds. Another runner completes the same distance in 45 seconds. Who has a greater speed and by how much?
Answer: We need to calculate the speed of both runners.
For the first runner:
Speed = \( \frac{400 \text{ m}}{50 \text{ s}} = 8 \text{ m/s} \)
For the second runner:
Speed = \( \frac{400 \text{ m}}{45 \text{ s}} \approx 8.89 \text{ m/s} \)
Comparing the two speeds shows that the second runner travels faster.
Let us find the difference between their speeds:
Difference = \( 8.89 \text{ m/s} - 8 \text{ m/s} = 0.89 \text{ m/s} \)
Thus, the second runner is faster by about \( 0.89 \text{ m/s} \).
In simple words: The runner who takes less time to cover the exact same distance runs faster. The second runner has a higher speed by 0.89 meters per second.

Exam Tip: When a calculation does not result in a whole number, round your answer to two decimal places and write "approx." or "approximately" next to it.

Question 3. A train travels at a speed of 25 m/s and covers a distance of 360 km. How much time does it take?
Answer: First, let us check the units. Since the speed is in meters per second, we should convert the distance from kilometers to meters.
Distance = \( 360 \text{ km} = 360 \times 1000 \text{ m} = 360,000 \text{ m} \)
Speed = \( 25 \text{ m/s} \)
Now, we calculate the time taken:
Time = \( \frac{\text{Distance}}{\text{Speed}} \)
Time = \( \frac{360,000 \text{ m}}{25 \text{ m/s}} = 14,400 \text{ s} \)
Next, let us convert these seconds into hours by dividing by \( 3600 \):
Time = \( \frac{14,400}{3600} \text{ hours} = 4 \text{ hours} \)
Thus, the train requires \( 4 \text{ hours} \) to finish the journey.
In simple words: We first turn kilometers into meters so the units match. Then, we divide the total distance by the speed to get the time in seconds and change it to hours.

Exam Tip: Always make sure all the units match before starting your calculations. If speed is in meters per second, distance must be in meters.

Question 4. A train travels 180 km in 3 h. Find its speed in: (i) km/h (ii) m/s (iii) What distance will it travel in 4 h if it maintains the same speed throughout the journey?
Answer:
(i) To find the speed in kilometers per hour:
Speed = \( \frac{\text{Distance}}{\text{Time}} = \frac{180 \text{ km}}{3 \text{ h}} = 60 \text{ km/h} \)
(ii) To find the speed in meters per second, convert \( 60 \text{ km/h} \) by multiplying by \( \frac{5}{18} \):
Speed = \( 60 \times \frac{5}{18} \text{ m/s} = \frac{300}{18} \text{ m/s} \approx 16.67 \text{ m/s} \)
(iii) To find the distance traveled in 4 hours at this speed:
Distance = \( \text{Speed} \times \text{Time} = 60 \text{ km/h} \times 4 \text{ h} = 240 \text{ km} \)
In simple words: Speed is distance divided by time. To turn km/h into m/s, multiply by 5/18. To find how far the train goes in a new time, multiply the speed by the new hours.

Exam Tip: Label each sub-part of your answer clearly (i, ii, iii) to match the question exactly. This makes it easier for the examiner to award full marks.

Question 5. The fastest galloping horse can reach the speed of approximately 18 m/s. How does this compare to the speed of a train moving at 72 km/h?
Answer: We have the horse's speed as \( 18 \text{ m/s} \).
The train's speed is given as \( 72 \text{ km/h} \).
To compare them, we must convert the train's speed into meters per second:
Train Speed = \( 72 \times \frac{5}{18} \text{ m/s} = 4 \times 5 \text{ m/s} = 20 \text{ m/s} \)
Since \( 20 \text{ m/s} \) is greater than \( 18 \text{ m/s} \), the train's speed is higher than the speed of the galloping horse.
In simple words: We convert the train's speed to meters per second. The train travels 20 meters each second, which is slightly faster than the horse at 18 meters each second.

Exam Tip: Never compare two speeds directly if they have different units. Change one of them so both speeds are written in either m/s or km/h.

Question 6. Distinguish between uniform and non-uniform motion using the example of a car moving on a straight highway with no traffic and a car moving in city traffic.
Answer: Uniform Motion: This happens when an object moves at a steady, unchanging speed along a straight path. It covers the exact same distance in each equal interval of time. For example, a car driving down an empty, straight road at a constant speed of \( 80 \text{ km/h} \) is in uniform motion.
Non-uniform Motion: This happens when an object's speed changes as it travels along a straight path. It covers different distances in equal intervals of time. For example, a car moving through crowded city streets has to speed up, slow down, and stop at traffic lights, which is non-uniform motion.
In simple words: Uniform motion means traveling at a steady, constant speed. Non-uniform motion means your speed keeps changing, like when you drive in heavy traffic.

Exam Tip: Key phrases like "equal distances in equal intervals of time" for uniform motion, and "unequal distances in equal intervals of time" for non-uniform motion, are critical keywords that examiners look for.

Very Short Answer Type Questions

Question. What is the SI unit of time?
Answer: The standard international (SI) unit for measuring time is the second. Its official symbol is written as \( \text{s} \).
In simple words: The basic scientific unit of time is the second, which we write as "s".

Exam Tip: Always write the symbol for seconds as a lowercase "s". Avoid writing "sec" or "sec." as they are not standard scientific symbols.

Question. What does a speedometer measure?
Answer: A speedometer is an instrument in a vehicle that measures its speed at any given instant. It displays this speed in kilometers per hour (\( \text{km/h} \)).
In simple words: It is the dial on your dashboard that tells you exactly how fast your car is traveling at that moment.

Exam Tip: Remember to include the unit (\( \text{km/h} \)) when defining what a speedometer measures.

Question. What is one oscillation of a pendulum?
Answer: One oscillation is the complete back-and-forth movement of a pendulum's hanging ball (bob). It starts from the center position, swings out to both extreme sides, and then returns to its starting center position.
In simple words: It is one full swing of the hanging ball from the middle, out to both sides, and back to the middle.

Exam Tip: Make sure your description includes the bob returning to its initial position to complete the loop, as a partial swing is not a full oscillation.

Question. Name a device used in ancient times to measure time.
Answer: In ancient days, people used early instruments such as sundials, water clocks, candle clocks, or hourglasses to track the passing of time.
In simple words: People used old tools like sundials and sand glasses to tell time before modern clocks were invented.

Exam Tip: Keeping two or three examples in mind, such as the sundial and hourglass, is highly useful for writing clear, multi-point answers.

Question. What is meant by uniform motion?
Answer: Uniform motion refers to the movement of an object traveling along a straight path at a perfectly steady speed, covering equal distances in equal blocks of time.
In simple words: It means moving in a straight line at the exact same speed without slowing down or speeding up.

Exam Tip: A simple definition must state that both the speed and the direction of the motion do not change.

Short Answer Type Questions

Question. What does a simple pendulum consist of?
Answer: A basic pendulum is made of a small, heavy metal sphere called a bob. This bob is tied to the end of a long, light string that hangs from a firm, unmoving support, allowing it to swing freely.
In simple words: It is just a tiny metal ball hanging from a string that can swing back and forth from a steady hook.

Exam Tip: Use the correct technical term "bob" instead of "ball" or "weight" to describe the swinging mass of the pendulum.

Question. What is the use of an odometer in vehicles?
Answer: An odometer is a device built into a vehicle's dashboard that records the total distance the vehicle has driven. It shows this distance in kilometers.
In simple words: It is the meter that counts how many kilometers a car or bike has traveled in its lifetime.

Exam Tip: Do not confuse the odometer (which measures distance) with the speedometer (which measures speed).

Question. Why do we use quartz or atomic clocks today?
Answer: We use quartz and atomic clocks because they are incredibly accurate. They can measure time precisely to tiny fractions of a second, which older mechanical clocks with gears could never do.
In simple words: Modern clocks are much better because they do not lose time and can measure very tiny splits of a second.

Exam Tip: Highlight that modern clocks offer much higher precision than older mechanical clocks that rely on swinging parts.

Question. How was time measured before mechanical clocks were invented?
Answer: Before clocks were created, people measured time by watching natural, repeating cycles like the sun rising and setting. They also built simple tracking tools like sundials, hourglasses filled with sand, flowing water clocks, or marked burning candles.
In simple words: People used the sun, shadows, running water, and sand glasses to keep track of time before modern clocks existed.

Exam Tip: Clearly separate your answer into natural events (like sunrise and moon phases) and early manual inventions (like sundials) to show deep understanding.

Question. Why is speed called a derived quantity?
Answer: Speed is known as a derived quantity because we cannot measure it directly on its own. Instead, we must calculate it by combining two base quantities - dividing the measured distance (in meters) by the measured time (in seconds).
In simple words: Speed is built from other basic measurements because you must calculate it using distance and time.

Exam Tip: Explain that "derived" means a measurement is built from basic, fundamental units like distance and time.

Descriptive Answer Type Questions

Question. How can you find the time period of a simple pendulum?
Answer: To find the time period, set the pendulum in motion and use a stopwatch to record how long it takes to finish 10 complete oscillations. Next, divide this total measured time by 10. This division gives the time taken for a single swing, which is the time period. Repeating this process a few times helps you double-check your work and shows that the swing time stays the same as long as the length of the string does not change.
In simple words: Use a stopwatch to time 10 full swings, then divide that number by 10 to find how long a single swing takes.

Exam Tip: Explain that measuring multiple oscillations (like 10 or 20) and dividing reduces timing errors compared to trying to time a single swing.

Question. Explain the importance of accurate timekeeping in sports and medicine.
Answer: Accurate timekeeping is highly important in many fields. In sports, modern races are so fast that athletes can win or lose by a tiny fraction of a second, so we must measure milliseconds to find the true winner. In medicine, special hospital devices like ECG machines must track extremely fast, small variations in a patient's heartbeat to spot heart issues. This exact timing is also essential for scientific studies and building advanced technology.
In simple words: Precise timing helps us find the true winner in fast races and allows doctors to check heartbeat details to save lives.

Exam Tip: Provide separate, clear examples for both sports and medicine to ensure you address all parts of the question.

Question. What is meant by non-uniform motion? Give one example.
Answer: Non-uniform motion happens when an object does not cover equal distances in equal blocks of time, meaning its speed is constantly changing. For example, a car driving through busy city traffic is in non-uniform motion because it must constantly speed up, slow down, and stop for traffic lights and pedestrians.
In simple words: If your speed keeps going up and down instead of staying steady, you are in non-uniform motion.

Exam Tip: A complete answer must include both a clear definition (changing speed over equal time intervals) and a practical example.

Question. What are the three main ways time is measured today?
Answer: Today, we measure time using three primary methods. First, we use mechanical clocks, which rely on moving gears and swinging pendulums. Second, we use quartz clocks, which track time using tiny, highly accurate vibrating crystals. Third, we use atomic clocks, which measure the incredibly stable vibrations of atoms. These methods range from mechanical (least precise) to atomic (most precise).
In simple words: We tell time using mechanical gears, vibrating quartz crystals, or the steady vibrations of tiny atoms.

Exam Tip: List the three types in order of their accuracy, from the least accurate (mechanical) to the most accurate (atomic).

Question. How does the length of a pendulum affect its time period?
Answer: The length of a pendulum has a direct effect on how long it takes to swing. As you make the string of the pendulum longer, the time period increases. This means a longer pendulum swings back and forth at a slower rate. This key rule is used by scientists to design clocks and study swinging movements.
In simple words: A longer string makes a pendulum swing slower, while a shorter string makes it swing faster.

Exam Tip: Clearly state the relationship: as the length of the pendulum increases, its time period also increases.

Exploring Questions

Question. Why is measuring speed important in everyday life?
Answer: Knowing how fast things move is very useful. It tells us how quickly an object is traveling, which helps us estimate when we will arrive, plan bus or train schedules, drive safely on the roads, and track athletic achievements. Without measuring speed, planning trips and navigating would be incredibly difficult.
In simple words: Knowing speed helps us figure out when we will arrive somewhere and helps us drive at a safe pace.

Exam Tip: Give a mix of examples - such as travel planning, road safety, and sports - to write a complete and high-scoring answer.

Question. Can the same pendulum be used at different places to measure time accurately?
Answer: A pendulum might not stay perfectly accurate if you move it to a different location. Its swing time depends on both its string length and the local pull of gravity. Because gravity changes slightly at different heights (like on high mountains or near the sea), the pendulum will swing at slightly different speeds in different places.
In simple words: Gravity is not exactly the same everywhere on Earth. Since gravity affects how a pendulum swings, it will keep time slightly differently in different places.

Exam Tip: Mention gravity as the key environmental variable that changes a pendulum's time period at different altitudes.

Question. How did Galileo’s observation of a lamp lead to time measurement discoveries?
Answer: Galileo noticed that a swinging lamp hanging in a church took the exact same amount of time to complete each swing, even as the swings became smaller. This observation inspired him to study pendulums, helping scientists discover that their repeating swings are highly consistent, which became the basic design for early mechanical clocks.
In simple words: Galileo saw a church lamp swing with a steady rhythm. This taught him that pendulums could be used to build reliable clocks.

Exam Tip: Note that Galileo's key discovery was that the swing time remains constant regardless of the swing's width (amplitude).

Question. What makes atomic clocks more accurate than pendulum clocks?
Answer: Atomic clocks are far more accurate because they measure the highly steady vibrations of cesium atoms. Unlike swinging pendulums, these atomic vibrations are not affected by changes in temperature, air pressure, or gravity, making them incredibly stable.
In simple words: Pendulums can be affected by heat or gravity, but atomic clocks rely on atoms that never change their rhythm.

Exam Tip: Mention "cesium atoms" and point out that they are unaffected by environmental factors like temperature and gravity.

Question. How does understanding motion help in space exploration?
Answer: Space travel relies entirely on precise calculations of speed and timing. By understanding how gravity and motion work, space scientists can navigate spacecraft, land robotic probes on other planets, and send missions across millions of kilometers with perfect accuracy.
In simple words: We must understand speed and path directions to successfully launch spacecraft and land them safely on other worlds.

Exam Tip: Highlight that space missions travel vast distances, where even a tiny error in speed calculations can cause a spacecraft to miss its target.

Question. How did people measure time before the invention of clocks, as explained in Class 7 Science Curiosity Chapter 8?
Answer: Before mechanical or digital clocks were made, people tracked time by watching repeating natural events like sunrise, sunset, and the changing phases of the moon. They also built clever early tools. These included sundials, which used moving shadows; water clocks, which measured a steady drip of liquid; and marked candle clocks. Later, they developed hourglasses with sand, which eventually led to the creation of gear-based mechanical clocks.
In simple words: Long ago, people used the sun, shadows, running water, and sand glasses to keep track of the passing hours.

Exam Tip: Categorize your answer into "natural events" (like moon phases) and "man-made tools" (like sundials) for a well-structured response.

Question. What is the relationship between speed, distance and time in Class 7 Science Curiosity Chapter 8?
Answer: The mathematical formula that connects these three concepts is:
Speed = \( \frac{\text{Distance}}{\text{Time}} \)
This equation lets us calculate how fast an object is traveling. If we know any two of these values, we can easily work out the third. For example, if a car covers \( 100 \text{ km} \) in \( 2 \text{ hours} \), we divide \( 100 \) by \( 2 \) to find its speed of \( 50 \text{ km/h} \). Because real journeys often involve changing speeds, we usually calculate the average speed for the whole trip.
In simple words: Speed is distance divided by time. If you know two of these numbers, you can easily use them to find the missing one.

Exam Tip: Memorize the triangle formula containing Speed, Distance, and Time to quickly rearrange the equation for any term.

Question. Why is the simple pendulum important in understanding time, as discussed in Class 7 Science Curiosity Chapter 8?
Answer: The simple pendulum is a key milestone in the history of timekeeping because it introduced the concept of steady, repeating motion called oscillation. Galileo discovered that a pendulum of a set length always takes the same amount of time to complete a swing, regardless of its weight. This discovery later allowed Christiaan Huygens to build the first reliable pendulum clock, establishing that accurate clocks must rely on consistent, repeating movements.
In simple words: The pendulum showed us that a steady, repeating swing is the perfect way to build an accurate clock.

Exam Tip: Be sure to mention the names of Galileo (who discovered the constant swing time) and Christiaan Huygens (who built the first pendulum clock).

Question. Why do we study time and motion in Class 7 Science Curiosity Chapter 8?
Answer: Studying time and motion helps us handle everyday tasks and organize our daily routines. It teaches us how to measure speed, plan our travel times, and track sports scores. By learning about clocks, speedometers, and pendulums, we understand the tools we use daily to measure how fast and how far things move.
In simple words: We study this to learn how to measure time and speed, which helps us plan our daily lives and trips.

Exam Tip: Give everyday examples like catching a train or checking a speedometer to show how this science is useful in real life.

Question. Is it difficult to understand the pendulum and its time period in Class 7 Science Curiosity Chapter 8?
Answer: Not at all! The concept of a pendulum is very simple. It is just a weight hanging from a string that swings back and forth. Its "time period" is simply the time it takes to complete one full swing. By doing simple activities, like timing a swinging key on a string, you can easily understand how it works.
In simple words: A pendulum is easy to understand. It is just a swinging weight, and its time period is the time it takes for one complete swing.

Exam Tip: To make this topic easy, remember that the time period only changes when you make the string longer or shorter.

Question. How can I do well in Class 7 Science Curiosity Chapter 8 without getting confused by formulas?
Answer: You can do very well by focusing on how the formulas connect to real-life situations. Instead of just memorizing "Speed = Distance / Time," think of speed as how many meters you can run in one second. Practice simple word problems step-by-step, and always write down the units to keep your calculations clear and organized.
In simple words: Do not worry about formulas. Just think of speed as how fast you move, and practice a few simple math steps.

Exam Tip: Make a small formula triangle on your rough sheet: put Distance on top, and Speed and Time on the bottom. This helps you remember all three equations easily.