You and Your Students!
Vicki Cobb, Education World Science Editor
A playground swing is really a timing device!
Setting the Scene (Background)
The nice weather is upon us and kids are spending more time outside. Why not send them to the playground with a problem to solve? All they need are swings and a provocative question.
Take a guess: In a swinging contest, who do you think would take longer to make two complete swings: a pumped-up-high swinger or a gently swaying rider?
Pose that question to your students. Get them to take a vote. It's ok if they guess wrong, because science is about making discoveries -- and you can make discoveries even when you guess wrong.
Some things must be seen to be believed, and this is one of them
For this experiment, your students can use two swings side by side or a single swing. If you use two swings, it is important that their ropes or chains be the same length. It is not necessary that the swingers be the same weight (although some kids might find this confusing and another experiment can be designed to see if the weight is a variable). Use a watch that times seconds or count off the seconds by saying, "One chimpanzee, two chimpanzee" and so on.
Get both swings going -- one high and fast, the other slow and low. Then have the riders quit pumping and let the swings move naturally. Time how long it takes for each rider to return to the place each started from. That means that if you start timing from the highest forward position, then you finish when the swing is again at that position.
Amazingly, there are no winners here. But there are no losers either. Big arc swings and little arc swings take exactly the same amount of time.
Behind the Scenes
To understand what is going on, you have to get physical. To a physicist, a swing is simply a pendulum. The time it takes for the pendulum to make one complete to-and-fro swing is called its period. The period of a pendulum is affected only by the pendulum's length. The distance it moves or the weight it moves has absolutely no effect.
Before mechanical clocks were invented, the Italian physicist Galileo discovered this law of pendulum motion. He noticed a swinging lamp in church and timed its period by using his pulse. He found that the period of the lamp remained the same even as it slowly decreased the distance it swung. This seemingly useless bit of trivia was later used by clockmakers. Pendulums became the basis for measuring time. (It's a good thing Galileo didn't get too excited by his discovery. If his heart rate had speeded up, the march of science might never have been the same.)
Article By Vicki Cobb
Copyright © 2005 Education World