Mark Schober

# Newton's Third Law Fidget Lab

Updated: Apr 29, 2021

I'm interested in curating a series of hands-on desktop activities that allow students to develop physical intuition for physics processes. The goal is for every pair of students to have access to the equipment at their desks or tables so that they can just fiddle with them during class sessions. I doubt that these will ever be as popular as fidget spinners, but I'm happy to repurpose the name as "fidget labs".

My colleague, Noah Segal, brought the idea of the third law cans to our school and then I built a class set of them. I first give the students a pair of cans and the students notice that the bottoms of the cans repel each other. I ask the students to compare the repulsion forces on each can. Their responses are generally mixed, but most agree that it's hard to tell which can is pushing harder. I give the students a third can that attracts to both of the first two cans. However, the third can weakly attracts to one can, and strongly attracts to the other can. I ask the students to compare the attractions between the cans. The notice the two different attraction strengths, but for one pair of attracting cans, the students can't determine whether one can is attracting harder than the other or not. When students team up and bring two of the third cans together, there is no interaction at all. I ask students to develop an explanation of what is in the cans that explains the forces they feel. This physical experience serves as a touchstone for the subsequent exploration and representation of Newton's third law.

### Building the Cans

I had a bunch of film canisters around, but you can find small bottles or containers on eBay. I bought a bunch of 3/4" fender washers, some strong neodymium magnets, and some ceramic magnets.

I first taped a neodymium magnet to my desk. I placed a canister on top of the magnet and then added one of the magnets inside the canister. (The goal is to make sure every magnet is oriented the same way in the cans.) I then filled the rest of the canister with discs of wood so that the magnet was firmly pressed against the bottom. You can fill the container with anything, from rice to dried peas, but you may want to run a band of tape around the canister to avoid potential messes. Colored electrical tape would be an easy way of distinguishing one type of can from another.

After one-third of the cans have a neodymium magnet in the base and one-third of the cans have a ceramic magnet, for the last third of the cans, place a fender washer in the bottom. It's these cans with no magnet at all that result in the attractive forces when brought near a magnet can and no interaction when brought to another washer can.