# Unknown Force?

1. Aug 16, 2007

### tommyburgey

Ages ago in my physics classes my teacher would have a large metal motor that would spin very fast, it was attached to a metal board. When he asked people to hold it it felt really stange and would pull you in apparently random directions. When it didnt spin it was very heavy but when it was spinning it's weight felt as though it changed, but I can't imagine it weighing less on scales. He would always say that he wanted to know what the force was, does anybody know about it or have any links?
(sorry if this was a nooby question)
Thanks, Tom.

2. Aug 16, 2007

### FredGarvin

Did it feel weird if you were holding it perfectly still or when you tried to move it in any way?

I can't really tell by your description, but it seems like you had a good sized gyroscope in your hands.

3. Aug 16, 2007

### tommyburgey

Both, I think. It wasn't a gyroscope though as there was only one direction of rotation (it was basically a spinning metal wheel).

4. Aug 16, 2007

### Nesk

This sounds very much like the moment of momentum, L, and the force you mentioned would probably be better described as a resistance towards the force you apply when trying to change the orientation of the disc. It is the same thing that makes bicycles more stable than you'd expect at first glance.

Every bit of the rotating disc is a moving mass and it thus carries a momentum. Collectively these momenta constitute the moment of momentum. Changing the orientation of the disc means chaning the movement of each bit, and thus the moment of momentum, which is conserved in an isolated system. This requires some work.

5. Aug 22, 2007

### «John»

also, you holding the spinning whatever it was, your hand isnt perfectly steady, so the momentum of the spinning object would move your hand seemingly random because the direction it(the mometum) is moving keeps changing (not sure if its true, but it sounds right =) )