The net force (Fn) acting on the bookshelf is the difference between forces that are acting to it in opposite directions. In the given above, the frictional force, 2.90 N, always acts opposite to the force applied to it, 35 N.
Fn = 35 N - 2.90 N = 32.1 N
Thus, the net force acting to the bookshelf is 32.1N.
