Wnet = ΔKE By this equation, if I lift a 1 kilogram book at rest from the ground and place it to be at rest on a table 10 meters above the ground, no net work has been done on the book. (Its kinetic energy before and after is zero.) However, its potential energy has changed by mgh or 1kg * 9.8m/s2 * 10m = 98J. Therefore its total mechanical energy (KE + PE) has increased but no net work has been done. By this reasoning the work-energy theorem implies that no net work needs to be done to increase or decrease the mechanical energy of an object. This is counter-intuitive to me. Is not 98J of work required to increase the total mechanical energy of a system by 98J? Is there a law that relates work to the change in mechanical energy of a system?