Physics & Physical Science Demos, Labs, & Projects for High School Teachers

The Spool and String and Torque

Posted on: March 18, 2009

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.

String coming off the bottom of the spool

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.

Torque on spool

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.


4 Responses to "The Spool and String and Torque"

I know that if you pull it to the right it will move right that fact is already established. But, what about the direction of the spin when you apply the force from the bottom? Why is it in the opposite direction to the torque applied by the string? Well, it must be due to some other force, which is friction. Then it must be that the torque from the force of friction is greater than the torque from the force applied. But, the whole point of why I ask a series of questions that I think I know the answers to is to ask one question… Why friction is in that direction? It boggles my mind.

Additionally, if you raise the string at an angle from the bottom, there is a point where the spool turns in the opposite direction of the force applied, which raises even more questions.

Friction is always in the direction opposite of motion, otherwise friction would cause acceleration.

Yes, if you lift up, at some point the vertical component of the pulling force will reduce the downward force to the point where the spool slips in place.

If you look at the discussion on inclined planes and friction (, you will see how an applied force can increase or decrease the normal force. The frictional force is directly related to the normal force, so if you have hardly any force pushing down on the surface, it slips much easier.

i want to know more about this spooling effect please help me…………….

I wouldn’t call it a “spooling effect,” it’s just torque causing a rotation about a point. I’m not sure what kind of help you are asking for, can you be more specific?

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