I know that [itex]F_f=\mu N[/itex] where mu is the coefficient of friction. I'm not concerned whether it's kinetic or static now, but I have a problem with the normal force. If I have two normal forces (for example, pushing a mop you have the normal from gravity and the normal from the angle at which the force is applied) how do I figure out the frictional force? Would I add the two normal vectors together? Or is the rule only to use the normal force from gravity?