The Spool and String and Torque
Posted March 18, 2009on:
This is one of my favorite simple demonstrations. I have a plastic spool that came from a pack of rope lights. The spool is about a foot across and I use it regularly to demonstrate a constant horizontal velocity.
When we study torque, I attach a pink mason line to the spool and wind it up. The question for the kids is, “What happens when I pull on the string?” Now, if it is unwinding from the top, obviously the spool is going to go in that direction. In the case of the drawing on the right, the spool is going to roll to the right.
But what happens when the spool unwinds from the bottom? Most of the kids think it is going to move to the left. A few will guess that it will stay in place, unwinding and slipping at the same time. Only a few think it will move to the right.
It does in fact move to the right. If we consider the point at which the spool meets the table our fulcrum, then we have a torque causing clockwise rotation according to the drawing at the right. The force is going to wind the string around the spool. Don’t believe me, try it for yourself.