SCIENCE CLASS- 7
CHAPTER-8 (Measurement of Time and Motion)
Let Us Enhance Our Learning
1. Calculate the speed of a car that travels 150 metres in 10 seconds. Express your answer in km/h.
Answer:
Speed = Distance ÷ Time
= 150 m ÷ 10 s
= 15 m/s
To convert m/s into km/h:
= 15 × 3.6
= 54 km/h
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:
Speed of first runner = 400 ÷ 50 = 8 m/s
Speed of second runner = 400 ÷ 45 = 8.89 m/s
The second runner has the greater speed.
Difference in speed = 8.89 − 8 = 0.89 m/s
Therefore, the second runner is faster by 0.89 m/s.
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:
Distance = 360 km = 360000 m
Time = Distance ÷ Speed
= 360000 ÷ 25
= 14400 s
= 14400 ÷ 3600
= 4 hours
4. A train travels 180 km in 3 h. Find its speed in:
(i) km/h
Speed = 180 ÷ 3 = 60 km/h
(ii) m/s
60 km/h = 60 × 1000 ÷ 3600
= 16.67 m/s
(iii) What distance will it travel in 4 h if it maintains the same speed throughout the journey?
Distance = Speed × Time
= 60 × 4
= 240 km
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:
Convert the train's speed into m/s:
72 km/h = 72 × 1000 ÷ 3600
= 20 m/s
Horse speed = 18 m/s
Train speed = 20 m/s
Difference = 20 − 18 = 2 m/s
Therefore, the train is faster than the horse by 2 m/s.
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: When an object covers equal distances in equal intervals of time, it is said to be in uniform motion. A car moving on a straight highway without traffic can maintain a constant speed and is an example of uniform motion.
Non-uniform Motion: When an object covers unequal distances in equal intervals of time, it is said to be in non-uniform motion. A car moving in city traffic frequently speeds up, slows down, or stops due to signals and vehicles on the road. Therefore, it shows non-uniform motion.
7. Data for an object covering distances in different intervals of time are given in the table. If the object is in uniform motion, fill in the gaps.
| Time (s) | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 |
|---|---|---|---|---|---|---|---|---|
| Distance (m) | 0 | 8 | 16 | 24 | 32 | 40 | 48 | 56 |
Answer: The missing distances are 16 m, 48 m, and the missing time is 40 s.
8. A car covers 60 km in the first hour, 70 km in the second hour, and 50 km in the third hour. Is the motion uniform? Justify your answer. Find the average speed of the car.
Answer:
The motion is non-uniform because the car covers different distances in equal intervals of one hour.
Total distance = 60 + 70 + 50 = 180 km
Total time = 3 h
Average speed = 180 ÷ 3
= 60 km/h
9. Which type of motion is more common in daily life—uniform or non-uniform? Provide three examples from your experience to support your answer.
Answer:
Non-uniform motion is more common in daily life because objects rarely move at a constant speed for long periods.
Examples:
- A bus moving through city traffic.
- A cyclist riding on roads with turns and obstacles.
- A person walking, sometimes speeding up and sometimes slowing down.
In all these cases, the speed keeps changing with time.
10. Data for the motion of an object are given in the table. State whether the speed of the object is uniform or non-uniform. Find the average speed.
| Time (s) | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Distance (m) | 0 | 6 | 10 | 16 | 21 | 29 | 35 | 42 | 45 | 55 | 60 |
Answer:
The object does not cover equal distances in equal intervals of time.
Therefore, the motion is non-uniform.
Total distance covered = 60 m
Total time taken = 100 s
Average speed = 60 ÷ 100
= 0.6 m/s
11. A vehicle moves along a straight line and covers a distance of 2 km. In the first 500 m, it moves with a speed of 10 m/s and in the next 500 m, it moves with a speed of 5 m/s. With what speed should it move the remaining distance so that the journey is complete in 200 s? What is the average speed of the vehicle for the entire journey?
Answer:
Total distance = 2 km = 2000 m
Time taken for first 500 m = 500 ÷ 10 = 50 s
Time taken for next 500 m = 500 ÷ 5 = 100 s
Total time used = 50 + 100 = 150 s
Remaining distance = 2000 − 1000 = 1000 m
Remaining time = 200 − 150 = 50 s
Required speed = 1000 ÷ 50
= 20 m/s
Average Speed:
Average speed = Total distance ÷ Total time
= 2000 ÷ 200
= 10 m/s