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What is a simple pendulum, and how is it used to measure time?

Class 7 ScienceNCERT chapter: Measurement of Time and Motionइसे हिंदी में पढ़ें

Exam-ready answer

A simple pendulum is a small heavy ball, called the bob, hung by a thread from a fixed support. When pulled aside and released, it moves to and fro, which is oscillatory motion. The time taken for one oscillation is its time period. For a given length it stays the same, so equal swings can be counted to measure time.

Explanation

To make a simple pendulum, tie a small metal ball or a stone to one end of a thread and fix the other end to a stand so that it hangs freely. When the bob is at rest, it is in its mean position. Pulling the bob slightly to one side and releasing it makes it swing to and fro. This regular to-and-fro movement is called oscillatory motion.

One oscillation is complete when the bob moves from one extreme position to the other and comes back. The time taken for one oscillation is the time period. To measure it, time 20 oscillations with a stopwatch and divide by 20; this reduces the error of starting and stopping the watch. The time period depends on the length of the thread: a longer pendulum swings more slowly and has a longer time period.

For a pendulum of fixed length, the time period stays almost the same, whether the swing is a little wider or narrower and whether the bob is heavy or light. Because every swing takes an equal time, the pendulum became a reliable timekeeper, and pendulum clocks count its oscillations to move their hands. The SI unit of time is the second (s).

Points to remember

  1. Simple pendulum: a bob suspended by a thread from a fixed point.
  2. Oscillatory motion: the regular to-and-fro motion of the bob about its mean position.
  3. Time period: the time taken by the pendulum to complete one oscillation.
  4. Time period = total time for 20 oscillations ÷ 20; its SI unit is the second (s).
  5. The time period depends on the length of the pendulum, not on the mass of the bob.

Exam tip

Draw a labelled diagram showing the stand, thread, bob, mean position and both extreme positions. Write the definition of time period and the method: time 20 oscillations and divide by 20, with the unit second (s).

Questions students also ask

Does the mass of the bob change the time period?
No. If the length of the thread stays the same, a heavier bob and a lighter bob take almost the same time for one oscillation. Changing the length of the thread, however, does change the time period.
Why do we time 20 oscillations instead of just one?
One oscillation takes only a second or two, so a small delay in starting or stopping the stopwatch causes a large error. Timing many oscillations and dividing by their number spreads this error out and gives a more accurate time period.
How was time measured before clocks?
Before mechanical clocks, people used sundials, which showed time by the shadow of a stick, water clocks, which measured time by water flowing into or out of a vessel, and sand clocks, in which sand flowed through a narrow neck.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.