NCERT Solutions for Class 7 Science Chapter 8 Measurement of Time and Motion (PDF) – 2027

By watching events in nature that repeat themselves after definite intervals of time — the rising and setting of the Sun, the phases of the Moon and the changing seasons. First, people made calendars (one day = one cycle of sunrise and sunset). To measure smaller intervals within a day they made sundials, water clocks, hourglasses and candle clocks:

Yes. Steps to make a simple water clock:

1. Take a used transparent plastic bottle (½ litre or larger) with its cap and cut it into two, roughly in the middle [Fig. 8.5a].
2. Make a small hole in the cap with a drawing pin [Fig. 8.5b].
3. Place the upper part, with the cap on, upside down over the lower half [Fig. 8.5c].
4. Fill the upper part with water coloured with a few drops of ink [Fig. 8.5d].
5. As the water drips down, mark its level after every minute (use a watch) till all the water has dripped down.

Pour the water from the lower part back into the top part and watch the level of water dripping into the lower part. Every time the level touches a mark you had made, one more minute has passed.

Yes — it is the same thing. An eraser hung from a thread and set swinging is a simple pendulum; only the name and the bob are different.

Yes. The bob swings from one side to the other and back again, again and again, along the same path — that is oscillatory motion, and it is periodic because the path repeats after a fixed interval of time.

Yes, the time period is almost the same every time. Sample readings for Table 8.1 (time period = time for 10 oscillations ÷ 10):

Conclusion: the time period of a simple pendulum of a given length is constant at a place.

What to investigate: does the time period depend on the length of the pendulum, the mass of the bob, or how far the bob is pulled aside? Does a longer pendulum swing more slowly? Does a heavier bob swing faster?

Would all pendulums have the same time period? No. Pendulums of different lengths have different time periods; pendulums of the same length have the same time period at a place.

How to test: change one thing at a time — the length (same bob), then the mass of the bob (same length) — and measure the time period each time.

Yes, the time period changes. The longer the pendulum, the greater its time period — a long pendulum swings more slowly than a short one. Sample readings with the same bob:

No, there is no change. Sample readings with the length fixed at 100 cm:

The time period of a simple pendulum depends on its length but not on the mass of the bob.

One second (1 s). The second hand moves from one small division of the dial to the next in 1 s (60 divisions in 60 s), and there are no marks in between. So one second is the smallest interval of time that we can measure using this clock.

By comparing their speeds — the distance each runner covers in unit time (say, one second). Speed = distance ÷ time. The runner who covers more distance in unit time is faster, whatever the length of the race.

We mean a comparison of the distances moved in a given interval of time. Something is moving fast if it covers more distance in the same time than another object; it is slow if it covers less.

The one who is ahead of the others at that instant is running faster. All the runners started together from the same starting line, so they have all been running for the same time. The one who is ahead has therefore covered more distance in the same time.

It can be calculated if we know the total distance covered by the object and the time taken to cover it.

Since speed is distance ÷ time, its unit is a unit of length divided by a unit of time. The SI unit of speed is metre/second, written m/s.

Speed can also be expressed in kilometre/hour (km/h), when the distance is in kilometre and the time in hour.

Use the railway timetable: note the distance to the next stopping station and the difference between the departure time from your station and the arrival time at the next one, then divide.

Sample answer (nearest railway station: Bhopal Junction; illustrative values — fill in the values from your own timetable in the same way):

The train which has covered the maximum distance in unit time is the fastest train — that is, the one with the highest speed. In the sample table above, that is Superfast train A at 97 km/h.

The runners who kept the same speed were in uniform linear motion — they covered equal distances in equal intervals of time. The runners who sped up or slowed down were in non-uniform linear motion — they covered unequal distances in equal intervals of time. All of them ran along a straight road, so all the motions were linear.

Yes. In Grade 6 we learnt that when an object moves along a straight line, its motion is called linear motion — for example, a train moving on a straight track.

Train X. Train X covers equal distances in equal intervals of time, so it is in uniform linear motion, while Train Y is in non-uniform linear motion.

Change m/s into km/h (1 m/s = 3.6 km/h):

The second runner has the greater speed — the one who took less time for the same distance.

The speed is in m/s, so first put the distance in metre:

(i) In km/h

(ii) In m/s

(iii) Distance in 4 h at the same speed

Bring both speeds to the same unit before comparing.

The train is faster than the horse, by 20 − 18 = 2 m/s (that is 7.2 km/h).

Both cars are moving along a straight road, so both are in linear motion. The difference lies only in whether the speed stays constant.

No, the motion is not uniform. In equal intervals of time (one hour each) the car covers unequal distances — 60 km, 70 km and 50 km. In uniform motion, equal distances are covered in equal intervals of time.

Average speed:

Non-uniform motion is far more common in daily life. Uniform linear motion is an idealisation; we seldom find objects moving with a constant speed over long distances or for long intervals of time.

1. A school bus speeds up on open roads, slows down at speed breakers and stops at bus stops.
2. Walking to school — we walk slowly, then faster when late, and stop at crossings.
3. A ball rolled along the ground moves fast at first, then slows down and stops.

Work out the distance covered in each 10-second interval by subtracting successive distances.

The intervals of time are all equal (10 s each), but the distances covered in them — 6, 4, 6, 5, 8, 6, 7, 3, 10, 5 m — are not equal.

So the speed of the object is non-uniform.

Sample answer. Float a light bowl with a fine hole at the centre of its bottom on water in a bucket, start a watch, and note when the bowl fills and sinks. Repeat with holes and bowls of different sizes. Sample readings:

Result: a bigger bowl takes longer to sink; a bigger hole makes it sink sooner. A large bowl with a very fine hole sinks in about 24 minutes — one ghati.

Part 1 — Pulse rate: place two fingers on the inside of a friend’s wrist, below the thumb, count the beats for 30 s and multiply by 2. Repeat three times and take the average. Record each friend’s pulse rate at rest and after exercise:

Part 2 — Pulse as a clock: at 72 beats per minute, one beat ≈ 60 ÷ 72 = 0.83 s. So counting pulse beats can measure a short time, such as a friend’s 100 m race.

Repeat the activity with these controls: the readings come much closer to one another.

Yes — a swing is a large pendulum. The chains are the thread, the seat with the child on it is the bob, and the frame overhead is the rigid support.

Sample readings:

Findings: children of different weights on the same swing give almost the same time period. A longer swing has a greater time period.

Speed = distance ÷ winning time. Sample, using the winning times of the Tokyo 2020 and Paris 2024 Games:

The speed was fastest in the men’s 200 m (Paris 2024, 10.28 m/s). The 100 m and 200 m speeds are nearly equal (men about 10.2 m/s, women about 9.2–9.4 m/s), and the 400 m is clearly the slowest (about 9.2 m/s for men and 8.3 m/s for women).