# I can not assume the sense of the friction force?

1. Aug 16, 2008

### hyper

With all the mechanical forces in statics problems I can assume the sense and I might get a negative answer which implies I made the wrong assumption. But why can I not do this with the friction force when using the equation F=uN?

2. Aug 16, 2008

### Staff: Mentor

Because the direction of friction is not independent of other variables. For example, if you assume something slides to the right, then friction on it must point left.

Perhaps discussing a specific example would help, if you have one.

3. Aug 16, 2008

### merryjman

Besides, the equation you mention is not a vector equation, so you can't use it to find direction. It can't be a vector equation, because in ordinary cases the normal force and frictional force are perpendicular.