Walking the Spool

PIRA: 1K10.30

In this torque demonstration, the string is wound once or twice about the spool. The spool is then placed on a long table top or the floor with the string's free end passing underneath. This is a good time to ask the audience which direction the spool will roll when the string is pulled with a steady force. Most will be surprised to see it roll toward the demonstrator, winding the string up the rest of the way.

There are two opposing torques present: one due to static friction and one due to tension. Since the frictional force is exerted further away from the axis of rotation, it results in a greater torque than that of the string. The string can also be pulled in different directions with different results.