NCERT Solutions Curiosity Chapter 8 .1.1 A simple pendulum — Think Like a Scientist!

Book page 1108 Updated on2026-09-05

Q1.

Suppose you were Galileo experimenting with pendulums, what all would you investigate? What questions could you ask? Would all pendulums have the same time period? How would you test this?

Answer

A scientist starts by listing everything about the pendulum that could change the time period, and then changes them one at a time. The things that could matter are:

the length of the pendulum (support to the centre of the bob),
the mass of the bob,
the material of the bob (iron, stone, glass),
how far the bob is pulled to one side (the size of the swing),
the place where the experiment is done.

Questions worth asking: Does a longer thread swing more slowly? Does a heavier bob swing faster? Does the swing slow down as it becomes smaller? Is the time period the same in Delhi and on a hilltop?

Would all pendulums have the same time period? No. Pendulums of different lengths have different time periods. But all pendulums of the same length have the same time period at a given place, whatever the bob is made of.

How to test it — the fair-test rule: change only one thing at a time and keep everything else the same. To test length, use the same bob and only change the thread. To test mass, keep the same length and only change the bob. If you change two things together you can never say which one caused the difference.

Did you know? Galileo had no stopwatch. Sitting in a church, he timed a swinging lamp using his own pulse beat and found that each swing took the same time. That single observation led, after Huygens, to the pendulum clock.

Q2.

Repeat Activity 8.2 using the same bob but with pendulums of two or three different lengths. Does the time period change? If so, how does the length affect it?

Answer

Yes, the time period changes. A typical set of readings, all with the same bob:

Length of pendulum	Time for 10 oscillations	Time period	What we see
25 cm	10.0 s	1.0 s	Short pendulum — swings quickly
50 cm	14.2 s	1.42 s	Longer — slower
100 cm	20.0 s	2.0 s	Longest — slowest

How length affects it: the longer the pendulum, the greater its time period. A long pendulum swings slowly; a short one swings fast. Notice also that the time period does not simply double when the length doubles — going from 25 cm to 100 cm (4 times the length) only doubled the time period.

Why this is so useful: because the time period depends on length in a fixed way, a clockmaker can tune a pendulum clock. If Huygens' clock ran slow, the bob was raised a little (shorter pendulum, smaller time period, faster ticking). Pendulum clocks still have this adjusting nut under the bob.

Try this: a pendulum of about 1 metre has a time period close to 2 seconds — one second for each one-way swing. That is why grandfather clocks are tall.

Q3.

If changing the length influences the time period, does the bob's mass also matter? Test this by repeating Activity 8.2 with a fixed pendulum length but with bobs of different mass. Do you observe any change?

Answer

No change is observed. With the length kept fixed at 100 cm and only the bob changed:

Bob used (length fixed at 100 cm)	Time for 10 oscillations	Time period
Small iron ball (light)	20.0 s	2.0 s
Heavy metal ball	20.1 s	2.01 s
Stone of about the same size	19.9 s	1.99 s

The tiny differences are only measurement error. So the conclusion of the box is confirmed: the time period of a simple pendulum depends on its length but not on the bob's mass. All pendulums of the same length have the same time period at a given location.

Why the heavy bob does not lag behind: a heavier bob is pulled down harder by gravity, but being heavier it is also harder to get moving. The two effects cancel out exactly, so the swing takes the same time. This is the same reason a heavy stone and a light stone dropped together reach the ground together — another discovery associated with Galileo.

Check it yourself: keep the thread length measured to the centre of the bob. If you swap a small bob for a very large one without re-measuring, the length has secretly changed and you will wrongly conclude that mass matters.

Was this helpful? Report an error