Just a little confused, when work is done on an object it's energy increases right? (i.e. Work done on object = Change P.E + Change K.E + Work done by object) So how come when gravitational force does work on an object(i.e. it falls) the potential energy decreases instead. So its like Work done by gravitational force = Decrease in P.E + Increase in K.E = 0? Edit : Okay, thinking about it probably because of action-reaction pair with the Earth? So work done on body = Decrease in P.E + Increase in K.E + work done by body?