# F=Ma Problem Help

1. Sep 19, 2009

### euroazn

Hi, I'm practicing for the physics olympiad qualifier and I came across this problem:

A spaceborne energy storage device consists of two equal masses connected by a tether and rotating
about their center of mass. Additional energy is stored by reeling in the tether; no external forces
are applied. Initially the device has kinetic energy E and rotates at angular velocity wEnergy is
added until the device rotates at angular velocity 2w What is the new kinetic energy of the device?

I know that E=.5Iw^2, but why does Ioda decrease by a factor of two?

Thanks.

2. Sep 19, 2009

### Redbelly98

Staff Emeritus
Welcome to PF

I will decrease because the masses are drawn in closer together.

Also, conservation of angular momentum could be helpful here.

3. Sep 21, 2009

### euroazn

Wow! Only one hour! Impressive ~ And the solution is accurate too (conservation of momentum)

My first post was just a "test" question, so to speak (I wouldn't be trying for Physics Olympiad if I didn't know about conservation of momentum =P) and I'm really impressed with the speed. More (unfortunately harder) questions to come.

Thanks Redbelly.