# Question about kinetic friction force?

1. Sep 2, 2008

### aiklone1314

hello everbody...i got two question about kinetic friction force...
tat is

1. is kinetic friction force of an object always the same value?as the formula kinetic friction force = coefficient of kinetic friction force*normal , coefficient of kinetic friction force is always the same...is me right?

2.then if applied force = kinetic friction force,there is always constant velocity of the object regardless how fast is the velocity ,is it? means that an object with constant velocity,10km/h has the same kinetic friction force with another object with constant velocity,150km/h ?

2. Sep 2, 2008

### tiny-tim

Welcome to PF!

Hi aiklone1314! Welcome to PF!
Yes … KF = µN,

so KF is the same if N is the same.

But N can be more or less than usual … for example, if the object is sliding down a curve, the centripetal acceleration may make N greater or smaller.
Yes … on a flat surface, and with no vertical forces, KF (for the same object) does not depend on speed.

3. Sep 2, 2008

### rcgldr

It depends on the two surfaces involved. In many cases the higher the speed, the lower the coefficient of kinetic friction, because the minor collisions between peaks and valleys cause the objects involved to seperate slightly at a higher speed differential.