Spinning Disk Trick Solution

Key Takeaways

[Applause] So in the spinning disc trick, we saw that an asymmetrically weighted disc when spun actually flips so that the lighter side goes towards the bottom. Now this is a variation on something called the tippy top. A little spinning toy that spins upside down so that its heavy side actually lifts up. Now this toy was played with by far more accomplished physicists than me, people like Neils Boore and Wolf Gang Pie. So, I have a little confession to make that when I made this spinning disc trick video, I didn't actually know how it worked. So, when I said I don't want to give you the answer right away, what I actually meant was, "Can you guys help me out and figure out how this works?" Now, in the comments, there were 25 comments pertaining to centrial force and 56 comments related to centrifugal force. So, clearly, I need to do a video about those things. I think most of the time you guys were thinking an explanation like this. Rotational systems tend so that their moment of inertia increases. So I'm thinking that as this starts spinning like this and is uh wobbling around. You think about there's a difference moment of inertia of this thing spinning around compared to at the bottom where there's more of the meat at the top up the top. And intuition is that takes effort for the ice skater to bring her arms in. Whereas just relaxing it tends to increase moment of inertia. Tribal force pushes it out. Now an additional 52 comments mentioned inertia betting that that's my answer for everything. Unfortunately, I don't think any of these things explains really what's going on with the spinning disc. And so I had to set out and do my own investigations. So the first thing I did was I talked to the smartest physicists I know. What do you think it's going to do if I if I rotate? It's going to spin around and it's going to turn upside down. So, the mass is up the top. I know. I bug it if I know why. I filmed the disc in 100 frames per second and 300 frames per second slow motion. And we labeled some points along the disc so we could really see what was happening. But even then, it was tricky to figure out. I took it out to the ice rink to see what role the surface played in terms of the spinning disc. So, what is my final explanation? it is that when the disc is spinning, the first thing we should notice is that the angular momentum remains constant the whole time. So when I spin it clockwise from above, the angular momentum is pointing vertically downwards. And that remains the same even as the disc flips over. So what is causing the disc to flip? Well, if the disc wobbles a little bit, it's off center. So its center of mass about which it's spinning is not located directly above the point of contact with the surface. And so as the disc spins there's some sliding friction which acts to create a torque on the disc. Now that torque reduces the angular momentum in the three direction of the disc and increases it in the one two direction of the disc which causes the disc to turn over like this. And once the disc goes past that point, the frictional torque at the bottom continues to rotate the disc until it is vertical like that. So it's really friction that is responsible for flipping this disc over. So the heavy part goes up. Now, there was an excellent video response made by Zog from Beetlejuice, so you should definitely check that out. Why don't I have any hair in that? Anyway, it is an amazing and hilarious response which is actually very physically accurate. So, check it out when you get a chance. Now, I have my doubts that this explanation is really the whole answer because I noticed that if I spin it above the ground, the hole still seems to go towards the bottom. That's kind of odd. And when I took it to the ice rink, well, sometimes the disc would behave as expected. And sometimes it just did strange things where the hole would go down and then up and then down and then up. I think maybe it's a sign of procession or perhaps mutation, but those are things for another video. So until next time, keep spinning those discs.