Depending on the situation, the calculation of the normal force N might include forces other than gravity. If an object is on a level surface and subjected to an external force P tending to cause it to slide, then the normal force between the object and the surface is just N = m g + P y , where m g is the block's weight and P y is the downward component of the external force. Prior to sliding, this friction force is F f = − − P x , where P x is the horizontal component of the external force. Thus, F f ≤ ≤ μ μ N in general. Sliding commences only after this frictional force reaches the value F f = μ μ N . Until then, friction is whatever it needs to be to provide equilibrium, so it can be treated as simply a reaction.