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Time one swing. What changes it: the string, the bob, or the push?
Simple Pendulum lab · Last updated: 16 October 2026
You set a pendulum swinging and time one full swing. Then they change one thing at a time: a longer string, a heavier bob, a bigger push. Only one of these changes the timing much, and it is not the one most people guess.
What you'll discover
- The time for one swing depends mainly on the length of the string.
- A heavier bob swings with the same timing as a light one.
- Small and medium pushes give almost the same swing time.
The science in plain words
A pendulum is a weight, called the bob, hanging on a string from a fixed point. Pull it aside and let go, and gravity pulls it back towards the middle. It overshoots and swings up the other side, then back again. The time for one complete swing, there and back, is called the time period.
Galileo noticed that a pendulum keeps a steady rhythm, and later scientists worked out the rule. For small swings the period depends only on the length of the string and the strength of gravity. A longer string means a longer period: four times the length doubles the time. A pendulum about one metre long takes roughly two seconds for a full swing.
The weight of the bob does not matter. A heavier bob is pulled harder by gravity but is also harder to speed up, and the two effects cancel exactly. The size of the push also hardly matters for small swings. Bigger swings cover more distance but move faster, so the timing stays almost the same.
This steady rhythm is why pendulums were used to keep time in clocks for about three hundred years.
Where it fits
- India, NCERT: Class 7 science, Chapter 8: Measurement of Time and Motion
- United States: Grades 6 to 8
Common questions
What affects how fast a pendulum swings?
Mainly the length of the string. A longer pendulum swings more slowly. The weight of the bob makes no difference, and for small swings neither does how far you pull it back.
Does a heavier pendulum swing faster?
No. Gravity pulls a heavier bob harder, but it is also harder to get moving, and these two effects cancel. Light and heavy bobs on equal strings keep the same time.
What is the time period of a pendulum?
It is the time taken for one complete swing, from one side to the other and back again. Timing ten swings and dividing by ten gives a more accurate answer.
Related labs
- Bungee Toy: Hang a toy on a big spring and let go. Does a heavier toy bounce faster or slower?
- Double Pendulum Twins: Release two double pendulums almost exactly the same. How long do they stay twins?
- Resonance: Shake three toys on springs gently. Why does only one go wild?
See all physics labs or every Taz science lab.
Try it yourself
The Simple Pendulum lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.