# Homework Help: Can someone explain

1. Nov 12, 2007

### Sucks@Physics

When a rigid body rotates about a fixed axis al the points in the body have the same angular acceleration. But why does the body as a whole have the same angular acceleration. just kinda of trying to understand

thanks

2. Nov 12, 2007

### rl.bhat

But why does the body as a whole have the same angular acceleration. Body rotates when a torque is applied to it. A torque is given by rXmi*a = mi*ri^2*alpha where mi is the mass of a partical at a distance ri from the axis.And alpha is the angular acceleration which is constant for all paricals. Total torque = sigma(mi*ri^2*)alpha = I*alpha

3. Nov 12, 2007

### CompuChip

If I understand your question correctly: that's the name we give it. If all points in a body have the same angular acceleration (such that no deformations etc. occur) we say that the body (as a whole) has a certain angular acceleration. It's what we often do in (classical) mechanics: we speak of properties of a body or its center of mass (velocity, angular momentum, etc) considering it as an atomic object, forgetting it is actually built out of smaller particles which all have that property (technically, I should add "on average" here, but that's outside the point I'm making).