# Why does my class ring flip up when I spin it?

1. Dec 31, 2015

### ElijahRockers

Just curious. Not sure if this has been discussed already, and I'm not sure what the phenomenon is called, but here's a description:

I have a heavy gold class ring that I got from my alma mater. I put the ring face down on the desk, flat, and it can rest like that because the face of the ring is flush with the desk. The ring face is a flat oval-shaped face about 1cm in diameter.

Now I can 'spin' the ring, like you would do with a coin or a top. Very quickly, the ring, while spinning rapidly, somehow flips up so the face is no longer in contact with the desk, but is facing skyward, and continues spinning, barely losing any momentum at all. I have tried to watch the transition carefully, at some point the ring MUST be completely sideways right before it flips up, which is just seems kinda crazy to me.

Why? Probably something basic, but as an EE I don't do a lot of mechanics, and this has been a question rattling around in my head for a long time.

2. Dec 31, 2015

### ElijahRockers

Clearly the heavy side down is some kind of unstable state, which falls into the stable state of the heavy side being 'up' while spinning... it just seems counter intuitive that the center of mass should be at the top of the system. Or am I thinking about this incorrectly?

3. Dec 31, 2015

Staff Emeritus
Answer A: You went to the wrong school. My ring doesn't do that:

Answer B: This is a variant of the "Tippy Top". The full solution is complex, but here is an outline. If you perturb the top (I will call the ring a top, because it's clearer) the point of contact with the ground will not be the same as the rotational axis. This applies a torque to the top, which if large enough will flip it. This raises the center of mass, and the energy has to come from somewhere. The only source is the rotational kinetic energy of the top, so the top slows down. In the process, it loses angular momentum (ultimately, to the ground). Now it can't reverse the process - energetically it's allowed, but the top no longer has enough angular momentum to spin in its original orientation. So it continues to spin heavy-side up.

4. Dec 31, 2015

### ElijahRockers

I beg to differ!

Are you sure you're spinning it hard enough? What does it do instead, just keep spinning on it's face?

But surely that doesn't happen in one fell swoop, I can clearly see the ring does almost a slow transition from face down to face up. It must be some kind of periodic, recursive function, I assume

5. Dec 31, 2015

Staff Emeritus
If that bothers you, don't think about the direction the ring or top is spinning in. That will really mess you up.

6. Dec 31, 2015

### ElijahRockers

Oh god.

7. Dec 31, 2015

Staff Emeritus
My ring is very difficult to spin because of the flat face. And the motion of a top (even a non-tippy top) is quite complicated, with precession, nutation, polhodes, herpolhodes and invariable planes. I described the beginning and the end. The middle is complicated.

8. Dec 31, 2015

### ElijahRockers

So is it just the fact that it's a square face, as opposed to a elliptical face? Because my ring also has a flat face.

It's times like these that I wish I had a slow motion camera.

9. Jan 1, 2016