Sunday, January 27, 2019

Torsional pendulum

This was another one of those things I wrote on my "To Do" list and figured I should complete before it had been on there a year.

On the day my AP Physics C students learn about torsional (twisting) pendulums I had written myself a note that while they "got it" they were having problems visualizing it. I didn't know how to construct one so I started an internet search and found a lot of problems about them but not a lot of demos. I found one video that looked promising and took a still to help guide my trip to the hardware store. The description called it a chuck nut which didn't seem quite right. After asking for help on Twitter I got a response:

He was nice enough to offer some advice and a link to the part he used. Once I knew what the part was actually called it was much easier to find them. When looking for the right pinch vise I looked for ones sold individually (most are in sets) and checked the range that it could hold. Many can securely hold amazingly small pieces so you have to also check out the max that they can hold. I settled on these, fairly cheap and should be versatile.

When I got them I was able to set up a few different demos. I drilled a hole into a golf ball the size of the end of one of the pin vises and wedged it in. I didn't want to glue it in yet because I didn't know what else I wanted to make. I cut lengths of fishing line, nichrome wire and a steel wire about the same length. I added a red and blue line on the golfball 90 degrees apart so it would be easier to see how much it was twisting. One end of each wire could be put into one of the two pinch vises, the one without the golfball hanging from a ring stand clamp.

I showed students the golfball oscillating with the fishing line, nichrome and steel wire in class. A video of each is below. We did not calculate anything (the wires were really bent) but it worked as a qualitative experiment. 

Afterwards I tried making some more with different masses, a small brass mass and a large rubber stopper. The tricky part is attaching the steel cable so that it doesn't twist within the object. I added hot glue to the brass mass but its not as secure as I would like. I added a black line in sharpie to the mass so the oscillation was easier to see. For the rubber stopper I was able to stab the cable through the stopper, although drilling a hole may have been more precise. I added a white pushpin to the side so that we could see the oscillation better. For these last two versions I only needed one pinch vise at the top to hold the cable. 

All in all I really liked the way it turned out and it helped students to visualize what was happening with their problems. It can always be improved but at least I have another year until I need it again. 

1 comment:

Unknown said...

Almost any spring will work for a torsion pendulum. I hang a 1 kg cylindrical mass from a hardware store spring, give it a twist, and let go. Can show period is independent from amplitude. Measure period, calculate I of mass as a solid cylinder and you can find the torsion constant. Then you can measure I of anything you hang from the spring using the period. Can estimate torsion constant by inserting dowel in end of spring and twisting it a known amount until you get a decent amount of force to measure with a force sensor. Torsion constant is force x lever arm/angle.