The normal force is defined as the net force compressing two parallel surfaces together, and its direction is perpendicular to the surfaces. In the simple case of a mass resting on a horizontal surface, the only component of the normal force is the force due to gravity, where N = m g . In this case, conditions of equilibrium tell us that the magnitude of the friction force is zero, F f = 0 . In fact, the friction force always satisfies F f ≤ ≤ μ μ N , with equality reached only at a critical ramp angle (given by tan − − 1 ⁡ ⁡ μ μ ) that is steep enough to initiate sliding.