Odd Spinning-Top Kind of Phenomenon I Can’t Explain

I was goofing off at work today, and I constructed a little device from a push pin (like these: http://en.wikipedia.org/wiki/File:Transparent_pushpins.JPG) and plastic bottle cap (like three of the four of these: http://en.wikipedia.org/wiki/File:Gatorade_G_Series.jpg). What I did is I stuck the push pin into the middle of the bottle cap so that it creates a kind of bowl with a stand. That is, I stuck the pin in the center of the bottle cap so that the “handle” of the pin would be outside and the point would be inside if you put the cap back on the bottle. The device looks a bit like a bird bath, albeit one with a spike in the middle.

Having constructed this device, I then took to spinning it like a top with the push pin on the bottom and the cap on top. And what happens is that if I spin it clockwise, it (obviously) spins like a top clockwise for a while, until it falls over and starts spinning along the rim of the bottle cap with the push pin base more or less in a fixed position. At this point, after a second or two, the device stops spinning clockwise and starts spinning counterclockwise. My question to you is: why does this happen? How does this spinning-top-esque device change directions like that? It just seems odd to me. (Incidentally, the record so far for spinning without hitting the rim is 27 seconds.)

It sounds to me like you accidentally created a “rattleback”.

This youtube video shows a traditional rattleback.

I’ve got one sitting on my desk at work that I created out of a plastic spoon (with the handle intentionally broken off) and a couple of push pins, like the type that you used.

The wikipedia page explains why they only rotate one way.

The rattleback idea is fascinating, and I will probably end up buying one. It could be what’s going on with my device, but it seems different.

Here is what the device looks like in case the verbal description isn’t clear:

\          /    |
 \________/    _|_
               | | PIN
    CAP        |_|

\     |     /   
     | |    DEVICE

Take this drawing, and rotate it around its vertical axis and you get the device. It’s not elliptical in the slightest as far as I can tell. Also, it works for each direction of initial spin. I think it might be linked to the friction of the desk on the rim of the bottle cap, but I have no idea why that would cause the device to change direction after a while.

There is also the Tippe Top as shown here which has the property that it prefers to invert itself and spin upon its stalk. The first link has a pretty good explanation of the physics involved.

Okay, so I couldn’t follow the OP so I just made a top as described and spun it to see if its behaviour might allow me to understand the OP.

And it turns out it’s really quite simple. After a clockwise spin, while still standing up on the pin it develops a wobble that due to some sort of precession effect also proceeds in a clockwise direction. This is caused by the non-pointy bottom of the pin, and is the same sort of wobble you can get spinning a drinking glass on its bottom rim. Then, after the makeshift top falls onto the rim of the bottlecap, the clockwise spin of the cap drives the device in a counter-clockwise rotation around the pin. This makes it look like it’s reversing direction, but really the outside edge of the cap is turning clockwise the whole time.

Great, I was almost to the end of the internet and then BAM, this thread pops up. Now I have to start over.

Ah. Thanks for doing all that. It makes sense to me now.

This is still, however, more evidence in favor of my pet theory that nature is just weird.

Y’know, I had thought that QED was the best-supported theory in all of science, but now that you mention it, QED itself supports your theory.

El birdbath de la muerte!