Disobedient Spool

Effective display of axis of rotation

Updated on: 06/13/13 inertia

To illustrate the importance of locating the correct axis of rotation in any rotational situation.

Spool, string

Place spool on a table and pull string horizontally. The spool will wind the string up and move in the same direction. A suggested teaching device is to set up a problem with given dimensions, moment of inertia and applied force and to postulate that the spool will accelerate in the opposite direction to the force with an acceleration, alpha. Complete the problem obtaining the (apparent) puzzling negative value of alpha. Then bring out the spool and demonstrate that the physics as calculated is correct.