# Homework Help: Rigid rotational body

1. Nov 22, 2005

### 9danny

Does a body rotating about a fixed axis has to be perfectly rigid for all points on the body to have the same angular velocity and the same angular acceleration? Why?

2. Nov 22, 2005

### kleinwolf

Yes, because the transmission of movement in a non-rigid body takes place basically from atom to atom...hence if you are too far ayway from the axis, there are regions that have still not been reached by the moving atoms, which have not transmitted their movement to the outer ones....maybe there is better explanation

3. Nov 22, 2005

### dx

Lets say the object was not rigid. Lets say a piece of jelly was rotating. If the jelly was initially not rotating and gradually attained a constant angular velocity,i.e it had an angular acceleration, do you think a point nearer to the axis would initially have had a higer or lower angular velocity, or equal?

4. Nov 22, 2005

### 9danny

I would think that a point near the axis would have more angular velocity at the beginning.. but does this means that for all the point in the object to have the same angular velocity and angular acceleration they'd have to be rigid?

5. Nov 22, 2005

### kleinwolf

That's a good question i thought i made a mistake....After some relfexion, I would say : if the ang. speed. is constant, then the body does not need to be rigid...in fact i think, in a real case, the body will modify itself during ang accel. at the beginning, and then reach an equilib. but this equib. final state at constant ang. speed is different than the body before you started to put it in rotation (maybe not for jelly, but for soft chocolate)...so the answer is : if you want same constant ang. speed (hence not the instantaneous one in an accel. case), it does not need to be rigid.....but if you have an ang. accel, it needs....i think it's more correct, but there still lack some calculation, for example : does the ang. accel. to be everywhere the same need maybe not to be completely rigid, but some kind of special deformation are allowed....

Last edited: Nov 22, 2005