# Help with free body diagram for circular motion

1. Dec 4, 2007

### eggman2001

Suppose you have an object (lets call it block A) resting on the edge of a circular rotating disk and sitting static on the disk. I'd like to visualize a free body diagram for this showing forces.

These are the forces I believe are acting on it:
- Fg - Force due to gravity
- Fn - Normal force equal in magnitude to Fg but it the opposite direction
- Fc - Centripital force towards the center of the disk
- Ffs - Force of static friction equal equal in magnitude to Fc in the direction that is tangent to the disk and in the same direction as the rotation of the disk.

Is this correct? I'm also trying to understand why Fc and Ffs would be equal in magnitude as I saw in an example.

Any help would be appreciated.

2. Dec 5, 2007

### Shooting Star

It is the frictional force which is the centripetal force. Without friction, the body wouldn't stay at rest with respect to the disk.

When the disk starts to rotate, the frictional force instantaneously will be in the direction tangential to the rotation of the disk, and pointing toward the direction of motion of the block. But once the angular velocity has become constant, the direction of the frictional force is toward the centre of rotation.

This can be visualized by going to the rotating frame. There, the block experiences a centrifugal force acting radially outward, which has to be balanced by the frictional force acting radially inward.

3. Dec 5, 2007

### eggman2001

That makes sense. Thank you.