# Moving on a spinning chair initially at rest

1. Aug 22, 2015

### alec_grunn

• Member advised to use the homework template for posts in the homework sections of PF.
Hey guys, this is not really a h/w question but I thought this would be the best place to get a relevant answer.

So I was doing physics h/w last night and found that I could spin on my chair (a standard office swivel chair) by moving my arms in the opposite direction to the way I wanted to spin. Also, the chair was initially at rest.
This seems to violate the conservation of angular momentum unless:

A) I'm actually providing a torque by pushing off the air (although, I doubt this is the case since when I stand up and flap my arms about I hardly feel pushed in any way).

B) This is due to a conservation of angular momentum, since moving my arms in one direction is a change in angular momentum of one part of the system, so the rest of the system has to compensate. Sort of like how a cat can always land on its feet when dropped upside-down.

C) Or any other answer you can think of.

Cheers

2. Aug 22, 2015

### haruspex

Can you be more specific about what you did and what you observed?
If the chair rotated only while you were rotating your arms the other way then it is easily explained by conservation of angular momentum. If the chair continued to rotate after your arms became still then you will need to consider friction.

3. Aug 22, 2015

### rcgldr

You can change orientation, but changing angular velocity requires a differential in friction. For example if there's enough friction, and the angular acceleration of your arms is slow enough, then the chair doesn't rotate but instead applies a torque to the floor, coexistent with the floor applying an opposing torque onto the chair, resulting in a change in angular momentum of the chair with you in it. If you stop your arm motion quickly enough, then there wont' be enough friction to prevent the chair from rotating for a breif period.

4. Aug 23, 2015

### alec_grunn

@haruspex I moved my arms in a circular arc clockwise and the chair spun counterclockwise. When I did this fast enough, I could actually spin around in the chair while seeming not to push off anything. The chair didn't continue to rotate when my arms stopped, so is the best guess conservation of momentum?

5. Aug 24, 2015

### haruspex

Conservation of angular momentum, yes.