Dueling Pendulums

Set each of these pendulums in motion and watch some
very interesting interactions.

Level: Middle to H.S.

Time involvement: 10-15 minutes.

Pendulums have been around for centuries. Their regular motion is almost hypnotic. Galileo in the 17th century used a swinging candelabra in a church as a science experiment. By using the pulse in his arm he discovered that the time for a complete swing was unchanged despite how short or long the swing. He also discovered that the weight of the “bob” did not affect the timing. Only the length determined how long each swing lasted! The dueling pendulums (as shown in the photo) are of equal weight and length. Although not shown in the photo the two pendulums can be directly attached to the horizontal pipe. With them directly attached it will be observed that when they are started together they will stay in step with each other!

However, when the pendulums are instead supported by a string loosely strung across the frame the resulting behavior will be completely different. By starting only one pendulum it will appear to be independent of the other pendulum. But wait. After a few swings it will be evident that the first pendulum is slowing down and the formerly resting pendulum is now beginning to move. In a short time the first pendulum will come to a stop and the second pendulum will now be swinging at the full extent that the firsts one had been swinging a short time earlier. The action will repeat many times with the action passing back and forth between the two pendulums.

Discussion: Energy itself is invisible but we can easily see its effects. The two pendulums are passing their energy back and forth much as two people playing catch. The loosely strung string is the pathway for the energy. The key are passing their energy back and forth much as two people playing catch. The loosely strung string is the pathway for the energy. The key area to watch is the short region of string between the pendulums. If you look closely the leading pendulum will be “towing” the other pendulum for as long as it is moving. The moment it stops the action will reverse and the second pendulum will become the “leader” towing the first. Soooo, no magic, just simple mechanics.