# Confusion regarding direction of kinetic friction on inclined plane.

1. May 18, 2013

### AakashPandita

The formula for kinetic friction acting on a sliding body is

μkN

When the body is sliding with constant velocity down an inclined plane

fkkN

When the body is sliding up the inclined plane(say due to a force acting parallel to the inclined plane) with constant velocity

even then

fkkN

The direction of kinetic friction is opposite in the 2 cases

while the formula for fk is exactly the same.

If friction is a vector shouldn't the formula for kinetic friction account for its direction too?

Last edited: May 18, 2013
2. May 18, 2013

### jbriggs444

For an object that is moving over a surface, the direction of the frictional force is opposite to the direction of relative motion. The coefficient of dynamic friction dictates the magnitude of this frictional force.

For an object that is stationary on a surface, the frictional force will act to oppose any other forces on the object so as to preserve its state of rest relative to the surface. The coefficient of static friction imposes a maximum magnitude on this frictional force.

If the surface is accelerating, a term accounting for that acceleration may need to be added in the static case.

3. May 18, 2013

### AakashPandita

i edited my post a little. could you elaborate please? i am not able to understand how it answers my question.

4. May 18, 2013

### Staff: Mentor

The formula just tells you the magnitude of the friction force. You also have to know that the kinetic friction opposes the sliding between surfaces.

5. May 18, 2013

### AakashPandita

Okay..

So that means we would always have to know the direction of the body sliding against a surface to tell the direction of kinetic friction acting on it?

Last edited: May 19, 2013
6. May 19, 2013

### Staff: Mentor

Yes.

7. May 19, 2013

thank you!