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NCERT Solutions · Class 7 Science · Chapter 8

Chapter 8: Measurement of Time and Motion (Physics)

Answers to all "Let Us Enhance Our Learning" questions of Chapter 8, Measurement of Time and Motion (NCERT Class 7 Science, Curiosity, 2026-27): speed, units m/s and km/h, uniform and non-uniform motion, average speed, and time from distance-time tables. All 11 questions are answered, with the key answer highlighted.

Speed = $\frac{distance}{time}$ . Average speed = $\frac{total distance}{total time}$ . To convert, $1 m/s = 3.6 km/h$ and $1 km/h = \frac{5}{18} m/s$ .

Let Us Enhance Our Learning

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

Solution

$Speed = \frac{150 m}{10 s} = 15 m/s = 15 \times 3.6 = 54 km/h$

15 m/s = 54 km/h

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?

Solution

$v_{1} = \frac{400}{50} = 8 m/s, v_{2} = \frac{400}{45} = 8.89 m/s$

The second runner is faster, by $8.89 - 8 \approx 0.89$ m/s (exactly $\frac{8}{9}$ m/s).

The second runner, by about 0.89 m/s.

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

Solution

$360 km = 3, 60, 000 m$ .

$Time = \frac{360000}{25} = 14400 s = \frac{14400}{3600} h = 4 h$

14,400 s = 4 hours

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?

Solution

(i) $\frac{180}{3} = 60$ km/h

(ii) $60 \times \frac{5}{18} = 16.67$ m/s

(iii) $60 \times 4 = 240$ km

(i) 60 km/h (ii) about 16.67 m/s (iii) 240 km

The fastest galloping horse can reach a speed of approximately 18 m/s. How does this compare to the speed of a train moving at 72 km/h?

Solution

$72 km/h = 72 \times \frac{5}{18} = 20$ m/s. The train (20 m/s) is faster than the horse (18 m/s) by 2 m/s.

The train (20 m/s) is faster than the horse (18 m/s) by 2 m/s.

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.

Solution

Car on an empty straight highway: it can keep the same speed, covering equal distances in equal intervals of time: this is close to uniform motion.
Car in city traffic: it speeds up, slows down and stops at signals and crossings, covering unequal distances in equal intervals of time: this is non-uniform motion.

On an empty straight highway the car covers equal distances in equal times (uniform); in city traffic its speed keeps changing (non-uniform).

Data for an object covering distances in different intervals of time are given (time 0, 10, 20, 30, __, 50, __, 70 s; distance 0, 8, __, 24, 32, 40, __, 56 m). If the object is in uniform motion, fill in the gaps.

Solution

The speed is constant: $\frac{8}{10} = 0.8$ m/s, i.e. 8 m every 10 s.

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

Missing values: distance 16 m at 20 s; time 40 s for 32 m; distance 48 m at 60 s.

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.

Solution

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

$Average speed = \frac{60 + 70 + 50}{3} = \frac{180}{3} = 60 km/h$

Non-uniform (different distances each hour); average speed 60 km/h.

Which type of motion is more common in daily life: uniform or non-uniform? Provide three examples from your experience.

Solution

Non-uniform motion is far more common, because moving objects usually speed up, slow down or stop.

A bus or car moving on a city road with traffic and stops.
A child running in a playground, speeding up and slowing down.
A cyclist riding uphill and downhill, or a ball rolling and gradually stopping.

(Near-uniform examples are rare: e.g. a train moving steadily on a straight track for a short time.)

Non-uniform: e.g. a bus in traffic, a child playing, a ball rolling to a stop.

Data for the motion of an object: time 0, 10, 20, …, 100 s; distance 0, 6, 10, 16, 21, 29, 35, 42, 45, 55, 60 m. State whether the speed is uniform or non-uniform. Find the average speed.

Solution

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

$Average speed = \frac{60 m}{100 s} = 0.6 m/s$

Non-uniform (unequal distances in equal times); average speed 0.6 m/s.

A vehicle moves along a straight line and covers 2 km. In the first 500 m it moves at 10 m/s and in the next 500 m at 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?

Solution

First 500 m: $\frac{500}{10} = 50$ s.
Next 500 m: $\frac{500}{5} = 100$ s.
Time left: $200 - 50 - 100 = 50$ s for the remaining $2000 - 1000 = 1000$ m.

$Required speed = \frac{1000}{50} = 20 m/s, Average speed = \frac{2000}{200} = 10 m/s$

20 m/s for the last 1000 m; the average speed is 10 m/s.

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