If you are holding an object off the ground and let it go, we say that it's potential energy is transformed into kinetic energy as it falls, but the total amount of energy stays the same. However, the gravitational force is doing work on the object, which seems like it would increase the amount of energy in the object. Similarly, when we lift a stationary object up, we say that it's potential energy increases. Yet two forces were acting on the object as we lifted it. The force from our hand did positive work, but the gravitational force did an equal amount of negative work. So how does the energy increase? Basically, I've suddenly realized that I don't really understand the definitions of work and potential energy.