Physics Q (x post)

[MrMon]

Isn't this just a form of precession?

Wiki - Precession

[PairTheBoard]

[ QUOTE ]
Isn't this just a form of precession?

Wiki - Precession

[/ QUOTE ]

This may apply to the problem, but I'm not sure how it applies to my theory that the positions of polarity of the two magnets are out of opposite synch at the beginning. The problem I have with my theory is that I don't see why it would imply a bias in one direction or the other. For the clockwise spin, at the top the Back - pole is pulling the Front + pole to the left, while the Front + pole is pulling the back - pole to the right. I don't see any directional bias. And even if there were, the forces at the bottom would be working in just the opposite way.

As far as precession, it seems like you need 3 diminsions for that. Here, the rotation is fixed in 2 dimensions by the glass.

PairTheBoard

[MadScientist]

There is something more going on here than what has been posted. I have one the magnets here in my office stuck to a metal filing cabinet.
The exact same behavior is exhibited.
If you place it on top of the filing cabinet, it just spins in place.
If you put it on the side and spin it cw, it moves to the left in a circular motion, but then when it gets to the edge of the circle, IT spins BACK and moves to the right, stopping underneath of where it started. It will spin a little more there and then come to a stop (but it is still spinning (sometimes for a few rotations)).
It never moves back up though.
I think it is a precessive type motion but it comes from the combination of magnetic field and gravity for sure. There is a torque applied by gravity as well due to the finite width of the magnet. It may be the combination of these two orthogonal rotational effects that lead to the circular motion.

I don't get how it can move back to the right after it was moving left, the direction of rotation is the same in both cases. However when it is stopped moving left, its momentum is purely down at that point.

[PairTheBoard]

[ QUOTE ]
There is something more going on here than what has been posted. I have one the magnets here in my office stuck to a metal filing cabinet.
The exact same behavior is exhibited.
If you place it on top of the filing cabinet, it just spins in place.
If you put it on the side and spin it cw, it moves to the left in a circular motion, but then when it gets to the edge of the circle, IT spins BACK and moves to the right, stopping underneath of where it started. It will spin a little more there and then come to a stop (but it is still spinning (sometimes for a few rotations)).
It never moves back up though.
I think it is a precessive type motion but it comes from the combination of magnetic field and gravity for sure. There is a torque applied by gravity as well due to the finite width of the magnet. It may be the combination of these two orthogonal rotational effects that lead to the circular motion.

I don't get how it can move back to the right after it was moving left, the direction of rotation is the same in both cases. However when it is stopped moving left, its momentum is purely down at that point.

[/ QUOTE ]

I still don't see how gravity acts as torque for precession. It's like a bicycle rolling downhill. The precession principle for the bicycle comes into play when force is applied in a third dimension perpendicular to the plane of the rotating wheels. The bicycle tends not to tip over and rights itself due to precession. With the magnet on the side of the cabinet gravity is acting within the plane of the rotation, like the bicycle rolling downhill.

Seems like you're probably right though and I'm just not seeing it. Gravity clearly has something to do with it or you'd see the same effect on top of the cabinet. The back and forth motion has got to rule out friction as the culprit.

Doesn't a spinning magnet generate an electric current in the metal cabinet? And doesn't that electric current generate a magnetic field of it's own acting back upon the magnet?

PairTheBoard