Demos: 1J-12 Variable Torque

This demonstration illustrates the difference between torque and force. A long wooden rod is attached at right angles to a shorter rod. The demonstrator grips the short rod in the hands and a weight is placed at increasing distances from the hand along the long rod. As the weight is moved out, it becomes more and more difficult to hold the weight up.

The hands create a friction force that is everywhere tangent to the rod, so the total friction torque exerted by the hands is



and the torque produced by the hanging weight and rod is



and



Directions: Have a student hold the short rod in his/her hand and then place a small (500-g) weight on the first hook. It will be relatively easy to hold, but as the weight is placed farther from the hands, it becomes more difficult (and maybe impossible).

Suggestions for Presentation: This one needs a student volunteer, so that you are free to move the weight around and the class can watch the student strain to hold up the weight.

Applications: Lots of applications to tools, for example, where placement of the force and load is important. Can be related to simple machines (levers).

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