Coupled Pendula - 100 To 1 Mass Ratio

Purpose

To illustrate mechanical resonance.

Equipment

Two pendula hanging in series from a fixed point as photographed.

Suggestions

This idea is used to amplify the oscillation of a gravitational wave antenna bar so that it can then be sensed by a sensitive transducer or by laser interferometry techniques.

Images

g2-24

Description

The two pendula have the same length, but the mass of the upper bob is 100 times that of the lower bob. With the masses hanging motionless, gently tap the bigger mass. Its energy will couple to the smaller mass, causing the smaller mass to oscillate with a much larger amplitude. The energy then couples back to the larger mass, and the cycle repeats.◙

References

J. -P. Richard, Approaching the Quantum Limit with Optically Instrumented Multimode Gravitational-Wave Bar Detectors,