# Confusion regarding direction of kinetic friction on inclined plane.

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:

jbriggs444
Homework Helper
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.

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

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

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?

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Nugatory
Mentor
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?

Yes.

thank you!