Moving on a spinning chair initially at rest

In summary: When you move your arms in one direction, it changes the angular momentum of one part of the system, so the rest of the system has to compensate. This is similar to how a cat can always land on its feet when dropped upside-down. Alternatively, there may be a differential in friction between your body and the chair, allowing you to change orientation but requiring a change in angular velocity.
  • #1
alec_grunn
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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
 
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  • #2
alec_grunn said:
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.
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
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 brief period.
 
  • #4
@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
alec_grunn said:
@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?
Conservation of angular momentum, yes.
 

1. How does the direction of motion change when someone moves on a spinning chair initially at rest?

The direction of motion changes because of the conservation of angular momentum. When someone moves on a spinning chair, they exert a force on the chair in the opposite direction of their movement. This force causes the chair to rotate in the opposite direction, changing the direction of motion.

2. Why does the rotation of the chair slow down over time?

The rotation slows down due to the principles of friction and inertia. As the chair rotates, it experiences frictional forces from the ground and air resistance, which act to slow down the rotation. Additionally, the chair's inertia, or resistance to change in motion, causes it to gradually slow down.

3. Can the direction of rotation be changed while the chair is in motion?

Yes, the direction of rotation can be changed by exerting a force in the opposite direction of the current rotation. This can be achieved by pushing against the ground or air, or by using external forces such as another person pushing on the chair.

4. What factors affect the speed of rotation on a spinning chair?

The speed of rotation is affected by the initial force applied, the mass and distribution of weight on the chair, the amount of friction and air resistance, and the shape and size of the chair.

5. Is it possible to maintain a constant speed of rotation on a spinning chair?

It is difficult to maintain a constant speed of rotation on a spinning chair because of the factors mentioned above. However, it is possible to achieve a relatively steady rotation by carefully controlling the amount of force applied and minimizing friction and air resistance. This is commonly seen in professional figure skating performances on a spinning chair known as a "spinner."

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