Let us consider a lift which is moving with a constant acceleration with a man inside. Now we know that the man exerts a force which is different from his weight depending on the direction of the movement. Now, when we consider the free body diagram of the lift do we take into consideration the force applied by the man on the lift (actually lift floor) and the force applied by the lift floor on the man? What are the guidelines for resolution of this kind of questions while writing the free body diagram. Why is free body diagram named as such? A body in general is acted upon by a few forces and thus is not free.