I am a bit confuse about gravity and work Suppose a ball falls from one height to another, hi to hf When talking about the system BE the ball AND the earth, all the work done is internal, and no external work, change in KE = change in PE, right? But when talking about the system B of just the ball only, total energy of the system B increase? because external force gravity is doing work on the ball? Change in KE increase, I know that, .5mv^2, velocity increase But what about PE? if gravity is external force, then the ball doesn't have potential energy to start with at hi ? So potential energy are only for a system that includes earth? Am I looking at these... gravitational potential energy and work stuff all wrong?