Recall that Newton's second law is valid only for inertial frames and hence you must be careful on its application. The standard problem of a block sliding down an incline plane uses an immovable inclined plane. for its solution, a coordinate system attached to the incline plane. This frame is inertial since the included plane is not moving. This makes the problem straightforward. Solve it again with a movable inclined plane: A block of mass m is placed on a wedge of mass M and angle as shown in the figure. What is the value of the acceleration of the block and what is the normal force from the incline on the block?