I can not assume the sense of the friction force?

[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?

 

[Doc Al]

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.

 

[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.