momentum conservation doesnt acomplish in a tetherball

carloman7 · 2005 May 5th, 12:46

a tetherball of a mass 1kg that changes radius from 1 m to 100 m at a constant speed of 100 m/s will have a momentum of 100*1*1 initially and a final momentum of 100*100*1 kg*m*m/s

can anybody explain this?

carloman7 · 2005 May 5th, 13:11

take into consideration that the pole thickness could be consider 0 because you could achieve the pulling of the cable by gearing the spin with the pull so the tetherball would be making a perfect circle

kublai · 2005 May 5th, 13:38

The specification of *constant* speed of 100 m/s requires input of additional energy to achive when the radius is changed to 100m, thus creating the added momentum.

[Edited on 5-5-2005 by kublai]

carloman7 · 2005 May 5th, 17:23

http://www.physicsforums.com/showthread.php?t=66697 pay atention to what is stated by krab who says that the linear speed of a tetherball is constant

kublai · 2005 May 5th, 19:22

Quote:

Originally posted by carloman7
http://www.physicsforums.com/showthread.php?t=66697 pay atention to what is stated by krab who says that the linear speed of a tetherball is constant

Different setup than yours. The radius gradually got smaller and there was no additional input of energy, only the blow that started the ball.

Your setup says there is a step up in the radius from 1m to 100m, yet the only way to retain the speed is to impart more force at the moment the rope is extended.

The tether ball in krab\'s setup is wound around the pole so that as the radius gets smaller it apears to accelerate. It really just has less distance to travel in in each rotation, so it seems faster.

The total energy in that system is actually reduced by the friction of the air on the ball thus transfering some momentum to the air.

Different ballgame. :P