Spinning Disk Trick Solution

[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 Neil's Bohr and Wolfgang Pauli. 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 centripetal 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 they moment of inertia increases.

So, I'm thinking that as this starts spinning like this and is wobbling around, you think about there's a difference in moment of inertia of this thing spinning around compared to at the bottom where there's more of the mass at the top. The intuition is that it takes effort for the ice skater to bring her arms in, whereas just relaxing tends to increase the moment of inertia.

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. So, I had to set out and do my own investigations.

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 spin it? It's going to spin around and it's going to turn upside down so that the mass is at the top." 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.

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! 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 precession or perhaps nutation, but those are things for another video. So, until next time, keep spinning those discs!