# Homework Help: Torque on a rigid body

1. Dec 9, 2007

### miss photon

1. The problem statement, all variables and given/known data
prove that when the net force on a rigid body is zero, the torque about any line perpendicular to the plane of the forces is equal.

2. Relevant equations
$$\tau$$=I$$\alpha$$

3. The attempt at a solution
it is known that angular velocity, and hence angular acceleration about any line is the same for a given rotating rigid body.
implies, $$\alpha$$ is same about all lines.
if we accept the above statement to be true, then I comes out to be equal about every axis, which is a contradiction.

2. Dec 9, 2007

### Feldoh

If it's perpendicular the the cross-product will just be tau = |r||F|

Last edited: Dec 9, 2007