PURPOSE: This demonstration illustrates the following concepts: (1) balance of torques, (2) friction and the normal force, (3) finding the center of gravity.

DESCRIPTION: Balance the meter stick ASYMETRICALLY on the two rotators, and start the rotators in motion. Then balance the meter stick as close to SYMETRICALLY as you can and start the rotators. It will always fall off if the rotators rotate outward, and will always end up balanced with the CM in the center if the rotators rotate inward. Which direction it moves depends on very small asymmetries!! Click your mouse on the photograph above to see what happens when the rotators rotate INWARD with the meter stick initially balanced ASYMETRICALLY.

For other cases, click below:

Rotators rotate outward, meter stick starts asymetrically balanced.

Rotators rotate outward, meter stick starts symetrically balanced and falls to the left.

Rotators rotate outward, meter stick starts symetrically balanced and falls to the right.

SUGGESTIONS: Try different starting points. Try adding a 100 gram weight onto either end. (Please ask for the weight so we do not overlook it.) It always works. Ask your students what they think will happen before doing the original experiment. Ask them again after adding a weight to one side. See Question of the Week #215 and Question of the Week #216 for more suggestions on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1J10.30) See Demonstration Reference File for lots of information on this demonstration and variations thereof.

EQUIPMENT: Meterstick with rotator setup, as photographed, with extra 100 gram weight if requested.

SETUP TIME: None.