|LECTURE DEMONSTRATION MANUAL | Instructional Research Lab : ucla physics|
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M.14.3 Tricks with a Meter Stick
Hold a meter stick at two arbitrary places with your two index fingers. When you bring your index fingers together, they will meet at the center of the meter stick. You can show that this is a consequence of the two different friction forces resulting from the different torques exerted by your fingers. Amazingly, even if the coefficients of friction are very different, say by covering one finger with slippery chalk dust and the other with a sticky rubber glove, your fingers will still meet at the center. If a weight is placed on one end of the meter stick, the fingers will still meet at the balancing point, the center of mass.
Professor Peter Schlein suggests this further demonstration of torque: Hold one end of the meter stick between your thumb and finger as shown above, and slide your index finger in from the other end. The force on your sliding finger increases from half the weight of the meter stick when your finger is at the far end to the full weight when your finger is at the center and the whole weight of the meter stick balances on one finger. As you slide your finger beyond the center towards the held end, your thumb must now exert a downward force, and the force on your sliding finger becomes extremely large. You can calculate the torques by assuming that the weight of the sections of the meter stick act at the centers of the sections.