This is my take on a device designed by Steve Rea. This device ended up being produced and sold by Arbor Scientific. Arbor's Rotational Inertia Demonstrator works amazingly well and is very repeatable. I highly recommend it. The cost really is quite reasonable for such a well built device. However, I decided to design a 3d printable one because our current physics teacher doesn't have the funds to buy a class set.
String is wrapped around one of the pulleys with a weight hanging from the string. There are three different pulleys to provide different amounts of torque. With an extra set of hex nuts the washers can be positioned different distances from the center of rotation changing the moment of inertia. There are a lot of experiments that students can do to investigate rotational motion.
Diameters of the three pulleys:
Update: I uploaded a new version with stops to keep the bearings from being pushed in too deep.
This was designed in TinkerCad.
Printed with 3 perimeters.
Parts List: (about $10 in parts/device)
- 2 each Skateboard Bearings, 608ZZ 8x22x7
- 4 each 6" long 1/4" hex bolts (or 1/4" threaded rods if you want longer arms)
- 12 each 1/4" hex nuts
- 40 each Fender washers, 1/4" hole 1.25" diameter
The bearings need to be de-greased first. The grease protects the bearings from water and road grit, but it will keep the bearings from spinning freely. I used acetone since we had it in the chemistry supplies. I just dropped them in a small beaker for 20-30 minutes. I found I also had to take them out of the acetone and spin them a couple of times then drop them back in. You need two bearings. One in each end for full support.
If you want to avoid using acetone do a little googeling for other ways to de-grease bearings. There's lots of stuff related to fidget spinners kicking around right now.