# Simple centripetal force question

1. Dec 4, 2011

### sparkle123

I think the force of friction should act to the left, toward the rotational axis.
My physics teacher says that the force of friction is acting out of the page, so I am confused.
Thanks!

2. Dec 4, 2011

### cepheid

Staff Emeritus
Hi sparkle123!

The rock wants to stay right where it is (Newton's 1st law) and not move. If it could, it would do so, while the disk simply slid underneath it. However, friction prevents the two surfaces from sliding relative to each other. In other words, friction carries the rock "along with" the rotation rather than letting it get left behind. So, to prevent sliding, friction acts in the direction of motion. The direction of motion is tangent to the circle at the point where the rock sits. At the point where the rock happens to be sitting, this tangential direction is "out of the page."

3. Dec 4, 2011

### Stonebridge

I beg to differ. If the disk has constant rotational speed there is no tangential force needed on the rock. The rock is moving at constant speed in a circle, and as such requires a centripetal force. This force is provided by the friction between the disk and the rock. It's direction is always towards the centre of the circle.
You would only need a tangential component to the frictional force if the disk were subject to an angular acceleration and the rock moved with it.

4. Dec 4, 2011

### sparkle123

Thanks cepheid and Stonebridge! :)

5. Dec 4, 2011

### cepheid

Staff Emeritus
Yeah I see. If the rock were sitting on a disk at rest, and you fired up the motor (which is implicitly the situation I was thinking of), causing the disk to spin up, then both a tangential and radial component of the force would be needed to keep the rock moving in a circle. But once the disk reached its constant "cruising" speed, then only the radial (centripetal) force would be required. So, you were right and I was wrong. The frictional force provides the necessary centripetal force, and as such it points towards the centre of rotation (i.e. to the left at this instant).

Sorry for the confusion. To the original poster: assuming the disk is moving at a constant speed, then you were right and your teacher was wrong.