D2-13.HTML
D2-13: RACING PENDULA

PURPOSE: To illustrate in a counterintuitive way the effect of moment of inertia on rotational acceleration.

DESCRIPTION: Two physical pendula, one of which has a weight on its bottom end, are held in a horizontal position and released from rest simultaneously. Q: Which one will reach the bottom first, or will it be a tie. A: The one without the weight will accelerate faster and reach the bottom first. This can be a rather tricky question, requiring careful analysis by the student. Mislead them by pointing out that pendula of the same length have the same period!

In this apparatus the position of the weight can be adjusted and set using a thumbscrew. Q: Where must the bob be placed so that the two pendula will accelerate at the same rate and reach the bottom simultaneously? A: At one-third of the distance from the bottom end. The period of a physical pendulum is equal to that of a simple pendulum with two-thirds of the length of the physical pendulum.

SUGGESTIONS:

REFERENCES: (PIRA unavailable.)

EQUIPMENT: Two pendula on support, as photographed.

SETUP TIME: None.