ignoring friction, compare the potential energy of a book on a shelf with the kinetic energy the book will have just before it hits the floor, if it falls from the level of the shelf. for the purposes of this problem, let's agree that all objects have zero potential energy when they are at floor level. the kinetic energy is larger by about 9.8 j for each kg of book mass. these two energies are equal. the kinetic energy just before hitting the floor is about twice as great as the potential energy at the start of the fall. the potential energy at the start of the fall is about twice as great as the kinetic energy just before hitting the floor. the potential energy is larger by about 9.8 j for each kg of book mass.