Torque Lab

We just did this lab yesterday and today.  It took the whole 50 minute period yesterday plus about 20 minutes today.  The feedback I got from the students was really great.  We studied torque in the classroom first, solved some problems, then did this lab.  They felt that they understood torque after completing the lab.  I came up with the lab in the shower in the morning (all of my great ideas and life solutions appear when I’m in the shower) and I wrote the whole thing up in my prep period before their class.  So much for planning ahead.

The lab uses a ring stand and support.  I have pendulum attachments for ring stands, but a regular ring will work just fine.  Using a meter stick, string, and some weights, they first create a balanced system with no net torque.  They then calculate the torque in each direction to show it is balance.  They do this with two weights and then with three.

Next, they pick a location for one weight and have to determine where the second weight goes to balance the system.  They know they get it when the meter stick balances right away.  Again, they repeat this with three weights.

Lastly, they move the fulcrum so the stick becomes part of the problem.  You can read the details in the lab, I think it’s fairly clear.

My lab sheets are here, feel free to use it as-is or modify them:

torque-lab

torque-lab-data-sheet

The Spool and String and Torque

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.