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3 Coupled Pendulums

Coupled pendulums can be formed by two pendulums suspended from a common support. In this setup, energy from one swinging pendulum can be transferred through the medium connecting the two pendulums. As a result, one swinging pendulum will transfer energy to the second pendulum, starting it swinging. As the energy is transferred back and forth, one pendulum stops and the other swings, reversing the energy transfer. Through careful observation, a prediction can be made when one pendulum will stop and the other one will start. It looks a bit magical but it's not. It's all about the transfer of mechanical energy.

FIG 1

See for Yourself

Materials

 string, 2 × 8 inch (20 cm) pieces

 tape

 ruler

 scissors

 straw

 2 washers with holes, or any comparable weight

What to Do

1 Tie one end of each string to a washer.

2 Tape the free end of each string to the edge of a table. The strings need to be about 6 inches (15 cm) apart. The string lengths for both pendulums need to be the same.

3 Cut 6 inches (15 cm) from the straw, and then cut short slits in both ends of the straw.

4 At about 2 inches (5 cm) from the tops of the string, attach the straw. The string should slide into the slits on the ends of each straw. Make sure that the straw is parallel with the table edge and the lengths of the pendulums below the straw are equal.

5 Pull one of the pendulums toward you a short distance and release; it should swing under the table. Observe movement in both pendulums.

What Happened?

Pendulums of the same length have the same natural frequency, which is the number cycles or the back and forth movements the pendulums would make in 1 second.

When only one pendulum is pulled and released, the end of the attached straw also moves back and forth with the same frequency. With each swing, the straw is giving the second pendulum small tugs. With each tug, the second pendulum moves a little higher. This happens at the expense of energy being transferred from the first pendulum. Thus, one pendulum slows down as the other pendulum swings higher. At this point there is a reverse of energy transfer. One might wonder why this back and forth transfer doesn't continue forever. The swinging pendulums are at the same time losing energy to other forms of energy, including heat and sound (air vibrations). Eventually, both pendulums will come to a standstill.