Reducing radius of rotating object

[Lemonio]

i have a very basic question.
me and my dad were arguing.
he was saying that if you were spinning in outer space, (no gravity air ...) with your arms opened up, and then you move them to your sides you would spin faster.
i was saying that almost nothing you could do would change the way you were moving in outer space

 

[cesiumfrog]

listen to your father

.. better still, ask that he take you to a park swing, that you experience the physics for yourself. (This will give a very strong feel for how much air resistance is related.)

 

[Lemonio]

ok. but i was confused because i thought that in outer space with no air to act on you, you wouldn't be able to alter rotating speed. linear speed change i understand but i am confused about the rotating speed. oh whatever i will just agree

 

[Doc Al]

i was saying that almost nothing you could do would change the way you were moving in outer space

Your Dad is correct. While no amount of arm waving will change the motion of your center of mass--it will keep moving in a straight line at constant speed--you can speed up your rotational rate by pulling in your arms. While you can't change your angular momentum (just like you can't change your linear momentum), you can change your rotational inertia by moving your arms in or out. Pulling your arms in reduces your rotational inertia, which increases your rotational speed.

Note that this is the same effect that you see when ice skaters spin faster by pulling their arms in.

 

[Lemonio]

now i think i understand, but i thought that ice skaters had to do with air recistance, i guess it's not important

 

[Danger]

It happens in space quite frequently, Lemonio. That's why neutron stars and black holes rotate so much faster than they did as normal stars.