Swings Keep Time
- Physics
- Ages 8–11 · Grades 3–5
- Hands-on experiment
What you’ll find out
- A pendulum repeats its motion, so it can be used to keep time.
- The time of a swing does not depend on the weight, or on how far it swings.
- A longer pendulum swings more slowly.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
A pattern you can time
Tie a weight to a string and hang it up. Pull the weight to one side and let go. Gravity pulls it down, and it swings through the bottom and up the other side, then back again. The motion repeats, and things that repeat can be used to keep time. Time one swing and a small mistake matters a lot, so time ten swings and divide by ten instead. Now try a heavier weight. The time for a swing does not change!
Longer string, slower swing
One thing does change the time of a swing: the length of the string. A short pendulum swings quickly. A long one swings slowly. A pendulum about one metre long takes about one second for each swing, which is why tall old clocks were built with a long case. Each swing moves the hands by one second. If you make the string four times longer, each swing takes twice as long, not four times.
Curriculum links: NGSS 3-PS2-2