So, the masses cancel each other. The force you exert on the book is not the only force acting on the book. The acceleration of the book is determined by the net force acting on it. As you push down on the book, the upward contact force the table exerts on the book becomes larger, exactly enough to balance the weight of the book and the force you exert on the book. Thus, the net force remains zero, and there is no acceleration.
Cushioned mats serve to make a collision last longer.
When a gymnast falls off the apparatus, contact with the floor exerts a contact force on her. To avoid injury, this force should be as small as possible. The hard floor exerts a large force in a short time, while the cushioned mat spreads the collision time between the floor and the gymnast over a longer time, resulting in a smaller force required to bring her to a rest on the floor. At point A, the roller coaster has a large amount of gravitational potential energy. This potential energy is converted to kinetic energy of motion as the coaster rolls down the hill.