Take a few bamboo skewers, some blue-tac, some ball-bearings and some rare-earth magnets: Put them together and shoot a ball bearing across the room.
The hardest part of this process was getting hold of ball-bearings. It turns out that hardware shops no longer sell them. You can buy made-up bearings, but not the balls. Ebay provided 40 ball-bearings for the princely sum of $4.50 including postage from China. The magnets also came through Ebay, although they were from a shop in Queensland.
It turns out that larger balls would work better than the 8mm ones I bought, but we managed anyway. The practical problem we discovered is that the magnets hold the ball in the center of their flat surface (if you look carefully you can see that on the right of the picture). With a 10mm-diameter magnet and an 8mm ball that means the ball sits off the improvised track and so did not always follow the desired path. Adding more balls to the stack after the magnet saw the third ball in line sitting neatly on the track though. But lesson for those who might come after – try to match magnet and ball diameter.
I’m now wondering if it’s viable to 3D-print a more effective track. I’m also wondering if we could do this in a circle or, even better, hanging like a Newtons Cradle but from a center-point (and, no, I’m not envisaging perpetual motion, just a couple of rotations). Anyway, for the moment this is an excellent and simple demonstration of momentum and magnetism in action.
Even our small test-rig really accelerates the balls. We were starting the first ball with the tiniest push so it was hardly moving; the ball coming out of the third stage was flying along. The slowest-motion video we could manage (reduced down to 4-frames-per-second) still shows the moving balls hitting our second and third stages as a blur. Exciting science.
We got the idea from a nice CSIRO video that largely explains the science. My understanding is that you need to set this up so your inputs into each stage exceed the force holding the next balls in place by enough to create acceleration. So the input ball has two forces working on it: momentum from the push and the pull of the magnets. As it hits the stage, being pulled in by the magnets, the force is transferred through the magnets and the stack of ball-bearings (thank you Mr Newton). The final ball bearing has nowhere to transfer the force but into motion and is also least attracted by the magnet as it’s furthest away. It shoots off only to be captured by the next stage and the process repeats itself.