Multiple Choice- Angular Velocity

[terrib]

Homework Statement

Which one of the following statements is NOT true for a rotating CD undergoing angular acceleration?
A) The angular speed is the same at any point on the disk
B) The lnear speed is the same at any point on the disk
C) The angular accerlation is perpendicular to the centripetal acceleration
D) It may have zero angular velocity
E) The angular accerlation is the same for anyh point on the disk

Homework Equations

The Attempt at a Solution

I was thinking that it would be it (D) would have zero angular velocity, because if it has angular acceleration, and it does not say that it is constant, then we can assume it has a velocity right?

 

[Dick]

An object can be accelerating and have zero velocity. Think of a thrown ball at the top of it's trajectory. What's your next guess?

 

[terrib]

Oh would it be (E)The angular accerlation is the same for any point on the disk. Wouldn't that be false because the acceleration closer to the point in the middle of the disk would be faster than the outsides rotational speed.

 

[Dick]

You may be confusing angular acceleration with things like radial and tangential acceleration. And an acceleration is not a speed. You can't compare the two. Angular velocity is how fast the angle to the center of rotation is changing. Since the disk is rigid, it's the same for all points on the disk. Angular accelertion is how fast the angular velocity is changing. Now what do you say to (E)?

 

[terrib]

Oh, I understand about what you are saying, the velocity is different at different radi on the disk, so the acceleration would be changing too. So (A) makes sense as the answer, because the speed isn't the same at any point on disk, it is always changing. Hopefully this is the right answer, because I am really confused on this stuff if I am wrong again.

 

[Dick]

Sorry. But you are confused. I was trying to convince you A and E were both true. Draw a picture of the disk and label the center and a point on the disk. The angle is the angle between that center and the point. The rate of change of that angle and the acceleration of that angle are THE SAME for all points on the disk. On the other hand you said something quite correct. The SPEED isn't the same for all points on the disk, since, for example, the center isn't moving at all and the outer parts are. What speed are you talking about?