# Friction in circular motion

1. Oct 23, 2006

### ndogg

If an object is placed on a record that is spinning, what is the direction of the force of friction?

2. Oct 23, 2006

### Ja4Coltrane

Which way is he object accelerating?
Got it now?

3. Oct 23, 2006

### ndogg

The object itself is not accelerating because it remains still on the record. So is there 0 friction?

4. Oct 23, 2006

### Ja4Coltrane

remember that acceleration is change in velocity per unit time not speed. Velocity has direction involved! This is centripetal acceleration, so the direction of friction is directly inward. Think of riding in a car around a sharp curve.

5. Oct 23, 2006

### ndogg

So the direction of friction is in the same direction as the centripetal force (which is the center of the record)? Shouldn't it be in the opposite direction because there has to be an equal and opposite reaction?

6. Oct 23, 2006

### Ja4Coltrane

Good question. The object sitting on the record is moving in a circle. Now pretend that the record does not exist, but instead, some arbitrary force holds the object in a circular path. You know that this force must be the centripetal force because if it were not, the object would go off on a path tangent to the circle. This arbitrary force is the friction of the record. Now the reaction force is actually the friction from the object acting on the record, but this force is not great enough to alter the record and is therefore negligible.

7. Oct 23, 2006

### ndogg

So, in this case, friction = centripetal force?

8. Oct 23, 2006

### Ja4Coltrane

Yes, good job. Just so you know, if the record were actually accelerating with speed too, you would just have two components of acceleration perpendicular to each other, so you can use A squared plus B sq = Csq

9. Oct 23, 2006

### ndogg

Thanks! Problem solved.