Physics Demo Number: 208

Approximate Run Time: 5 min

Finding the Center of Mass of a weighted meter stick

Demo Description

By supporting a meterstick (which can have a weight placed anywhere on it) on two fingers, and gradually bringing the fingers together, one can locate the center of mass of the weighted meterstick.

As the two supports are brought closer to each other, they alternate between slipping and sticking (e.g. if at any time the left finger slips, the right sticks, and then the right finger will begin to slip while the left sticks and so on. The explanation for this involves concepts of friction, torque and equilibrium. )

Scientific Principles

Equipment

• Meter Stick

Equipment Location

Meter sticks are located in the demo room by the door to Room 4327.

Instructions

The reason that the fingers stick and slip alternately is because the maximum force of static friction between a finger and the meter stick is inversely proportional to the finger's distance from the center of mass of the weighted stick (this can be easily proved from equilibrium considerations).

A quick video of this demo, and a detailed explanation by Prof. Rhett Alain, can be found here