Several weeks ago in physics, we were discussing circular motion. At one point during the day, the class was discussing what the path of an object should be immediately after the forces causing the circular motion cease (e.g., after a string breaks). The answer that students were supposed to settle on was that the object moves in a straight path tangent to the circle–an answer that is consistent with Newton’s 1st Law. One student, however, wasn’t buying it. He thought that if you spin something fast enough you should be make it keep curving around, even after the forces have lost contact. I said, “I’m with you man. Science is all about skepticism and gathering compelling evidence and arguments. You have skepticism, now go find some compelling evidence and convince me”
Weeks later that student comes back with a coat hanger, and starts spinning the coat hanger around his finger. He says this, “If the path of my finger at the end of the coat hanger goes straight at the end of the hook, then I couldn’t possibly twirl this coat hanger around my finger. He points out specifically that it only works when he spins the coat hanger really fast-fast enough to make it all keep curving around the gap.”
What’s the physics here?
What’s your next move with the student?
The student is saying that when he stops trying to spin it faster, the coat hanger keeps spinning around his finger. I suppose I might ask him what he feels as the hanger is spinning. Does the force of the coat hanger pushing on his finger go away when he stops trying to spin it? Contrast this with the mass on a string, and the force of the strong on your hand definitely goes away when you let go of the string.
I think this is a cool problem. If he spins it fast and then pulls his finger out, it keeps spinning (conservation of angular momentum at that point) along with now traveling in a trajectory. Do you have to distinguish circular motion from rotational motion? It reminds me of a comment I put on one of Veritasium’s videos (the one about the huge spherical boulder with the low friction support). I said that if the particles on the surface are going in a circle, something has to be providing a centripetal force. For the boulder, that’s internal forces. But there’s no external centripetal force in that case.