Coupled Resonant Pendulums 2

1. Tape the two pencils to the edge of a table.
2. Cut two strings of equal length (about 8–12 inches [20–30 centimeters] works well) and tie a paper clip to each end.
3. Tie the other end of each string to the end of a pencil and adjust the knots so that you have two pendulums of equal length.
4. Attach two pennies to each paper clip.
5. With the scissors, shorten the drinking straw to about 6 in (15 cm), cut small slits along the sides of the straw, and use the straw segment to link the two pendulums together.

Pull one pendulum toward you a short distance and let go.

Notice that after a few swings, the second pendulum will begin to oscillate, or swing back and forth, with the same frequency as the first pendulum.

With each swing, the second pendulum will increase its amplitude, or the height of its swing. Eventually, the pendulums will swing in unison—the second pendulum will swing in resonance with the first one.

Every pendulum has a natural vibration cycle that depends only on its length. For example, a weight tied to the end of a 10-in (25 cm) length of string will complete one back-and-forth swing in about one second.

The two pendulums in this activity have the same natural frequency because you made them equal in length.

When you start the first pendulum oscillating, it makes the attached drinking straw twist back and forth with the same frequency. Each time the first pendulum completes a swing cycle, the twisting straw gives the second pendulum a tiny shove—like a parent pushing a child on a swing.

Because the straw is pushing with the same rhythm as the natural frequency of the second pendulum, the weight swings progressively higher and higher with each tiny push.

Imagine you're stopped at a traffic light when a loose door panel on your car begins to rattle loudly. What could make it vibrate energetically even though the car is at a complete stop?

Like swinging weights on a pendulum, your car door panel has a natural vibration frequency. In this case, the pistons, which move up and down in your idling engine, match the resonant frequency of your door panel.

Metal between the engine and the car door, like the drinking straw in this pendulum activity, transmits the pushes and pulls that eventually get the loose panel to shake violently. Each tiny motion of the car body will make the loose door panel vibrate harder and harder—until finally the amplitude of the vibrations are large enough to get your attention.