# Rotation of Rigid bodies

1. Jul 22, 2009

### Amar.alchemy

Hi All,

I don't know whether this question makes any sense or not anyhow i will ask. For example, consider a ball which is attached at the end of the string and let us assume that this ball rotates in the uniform circular motion. Then the reason it rotates in uniform circualar motion is its tangential velocity and radial accelaration(ie perpendicual component of accln).
Now consider a rigid body which rotates with constant angualr velocity about the fixed axis. My question is what makes the rigid body to rotate about the fixed axis?? kindly explain me.

2. Jul 22, 2009

### cepheid

Staff Emeritus
Are you asking where the centripetal force comes from? If we assume the body is rigid, we are saying that every atom or molecule within it is fixed in place and cannot move relative to the others (due to chemical bonds or whatever). So every point particle that comprises that object individually sweeps out a circle around the axis of rotation. The radius of said circle depends upon how far away that point particle is from the axis. These individual motions together constitute a rotation of the entire body.

3. Jul 22, 2009

### Amar.alchemy

Exactly...
So what make these point particles to go around the fixed axis??

4. Jul 22, 2009

### Cyrus

The imposed constraint via the string.

5. Jul 22, 2009

### Amar.alchemy

In rigid bodies where does the sting comes??

6. Jul 22, 2009

### Cyrus

I don't understand what you're asking.

7. Jul 22, 2009

### cepheid

Staff Emeritus
This was mean to be my answer to that question:

Every point is fixed in place relative to the others. Therefore, when you rotate the body, every point sweeps out a circle that is dictated by its (unchanging) position within the body. Do you understand?

8. Jul 22, 2009

### Amar.alchemy

Ya, thanks :)