(1) When inclined plane is at rest.
Normal reaction R = mg cos θ
Force along a inclined plane F = mg sin θ
ma = mg sin θ
a = g sin θ
(2) When a inclined plane given a horizontal acceleration ‘b’. Since the body lies in an accelerating frame, an inertial acts on it in the opposite direction.
Normal reaction R = mg cos θ + mb sin θ
and ma = mg sin θ – mb cos θ
a = g sin θ – b cos θ