# Spinning Top / Dancer

1. Aug 12, 2010

### Gerenuk

If a dancer who balances on one leg starts spinning, is having the center of mass above the point of contact enough, or does he need to assume a special shape?
I mean is the moment of inertia important provided the point of contact (feet) are not slipping?

2. Aug 12, 2010

### Dr Lots-o'watts

The center of mass has to be over the pivot point. The more the mass is concentrated near the axis, the faster the spinning for a given starting energy.

3. Aug 13, 2010

### Gerenuk

So wasn't so much the question.
I already assume the the COM is above the pivot point. The question is whether this is enough for a stable rotation.
I mean in free space there are only two special axis of rotation about which a stable rotation is possible. What about a spinning top? Can it's shape be arbitrary?

4. Aug 13, 2010

### rcgldr

Figure skaters are able to assume various positions while spinning, but the point of contact with the ice translates across the ice somewhat during a spin.

5. Aug 13, 2010

### Gerenuk

OK, but that's also not the question. The question is whether there are positions that don't work.

6. Aug 13, 2010

### GRDixon

Yes, I would think so, provided its spin axis passes through the center of mass.

7. Aug 13, 2010

### Dr Lots-o'watts

1. The lower the center of gravity, the more stable it will be. The reason is that small oscillations, whether intrinsic or external will not have as much tendency to displace the center of gravity over the pivot point, if it very low. In this sense, the shape will have an effect.

2. Also, the lighter the object, the less there will be friction at the pivot point, so the less energy it will waste.

3. You could also minimize energy loss by making it aerodynamic in the relevant direction.