Class 7 Science · Chapter 8

8. Measurement of Time and Motion

Complete solutions to Let Us Enhance Our Learning. CBSE / NCERT Class 7 Science (Curiosity).

Let Us Enhance Our Learning

1Calculate the speed of a car that travels 150 metres in 10 seconds. Express your answer in km/h.

Speed = Distance ÷ Time = 150 m ÷ 10 s = 15 m/s
1 m/s = 18/5 km/h
Speed = 15 × 18/5 = 54 km/h

The speed of the car is 54 km/h.

2A 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?

Speed of first runner = 400 ÷ 50 = 8 m/s
Speed of second runner = 400 ÷ 45 = 8.89 m/s (approx.)
Difference = 8.89 − 8 = 0.89 m/s

The second runner is faster by about 0.89 m/s.

3A train travels at a speed of 25 m/s and covers a distance of 360 km. How much time does it take?

Distance = 360 km = 360 × 1000 m = 3,60,000 m
Time = Distance ÷ Speed = 3,60,000 ÷ 25 = 14,400 s
14,400 s ÷ 3600 = 4 h

The train takes 4 hours.

4A 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 at the same speed?

(i) Speed = 180 km ÷ 3 h = 60 km/h
(ii) 60 km/h = 60 × 1000 m ÷ 3600 s = 16.67 m/s (approx.)
(iii) Distance = Speed × Time = 60 × 4 = 240 km

Answers: 60 km/h 16.67 m/s 240 km

5The 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?

Speed of train = 72 km/h = 72 × 5/18 = 20 m/s
Speed of horse = 18 m/s
Difference = 20 − 18 = 2 m/s

The train is faster than the horse by 2 m/s.

6Distinguish 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.

Uniform motion	Non-uniform motion
Example: a car on a straight highway with no traffic.	Example: a car moving in city traffic.
The car moves with a constant speed.	The speed of the car keeps changing.
It covers equal distances in equal intervals of time.	It covers unequal distances in equal intervals of time.
The car does not need to slow down or stop.	The car slows down, stops and speeds up again at signals and crowded roads.

7Data for an object covering distances in different intervals of time are given. If the object is in uniform motion, fill in the gaps in the table.

Speed = 8 m ÷ 10 s = 0.8 m/s. The object covers 8 m in every 10 s.

Time (s)	0	10	20	30	40	50	60	70
Distance (m)	0	8	16	24	32	40	48	56

8A 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.

No, the motion is non-uniform. The car covers unequal distances in equal intervals of time (1 hour each).

Total distance = 60 + 70 + 50 = 180 km
Total time = 3 h
Average speed = 180 ÷ 3 = 60 km/h

9Which type of motion is more common in daily life — uniform or non-uniform? Provide three examples from your experience to support your answer.

Non-uniform motion is more common in daily life.

A bus moving on a road stops at bus stops and signals.
A child cycling to school slows down at turns and speeds up on a clear road.
A person walking in a market keeps changing speed.

In each case, the speed keeps changing.

10Data for the motion of an object are given. State whether the speed of the object is uniform or non-uniform. Find the average speed.

The distances covered in every 10 s are 6, 4, 6, 5, 8, 6, 7, 3, 10 and 5 m. These are not equal. So the speed is non-uniform.

Total distance = 60 m
Total time = 100 s
Average speed = 60 ÷ 100 = 0.6 m/s

11A vehicle covers 2 km in a straight line. In the first 500 m its speed is 10 m/s and in the next 500 m its speed is 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 for the entire journey?

Time for first 500 m = 500 ÷ 10 = 50 s
Time for next 500 m = 500 ÷ 5 = 100 s
Time left = 200 − (50 + 100) = 50 s
Remaining distance = 2000 − 1000 = 1000 m
Required speed = 1000 ÷ 50 = 20 m/s
Average speed = 2000 m ÷ 200 s = 10 m/s

Required speed = 20 m/s; average speed = 10 m/s.

Exploratory Projects

Exploratory Projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.

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