In general, process for solving any statics problem with friction is to treat contacting surfaces tentatively as immovable so that the corresponding tangential reaction force between them can be calculated. If this frictional reaction force satisfies F f ≤ ≤ μ μ N , then the tentative assumption was correct, and it is the actual frictional force. Otherwise, the friction force must be set equal to F f = μ μ N , and then the resulting force imbalance would then determine the acceleration associated with slipping.