Hello, i was wondering if someone can help me out. I finished law of momentum, and learned that for the conservation of momentum to be applicable, the system must be isolated. An isolated system means no external forces, correct? if i am correct, then i dont understand the law of conservation of mechanical energy. it says that conservation of mechanical energy must be in an isolated system. this is K + U (initals) = K + U (finals) but the U term is mgh, which has gravity in it. isnt gravity an external force? so how is this an isolated system?