I know that if gravity is the only thing that does work on a "particle", the change of kinetic energy of that "particle" will be equal to the work done by gravity(∆K = -∆Ugrav). I know that "∆K = -∆Ugrav" is true for a "particle". But how is that true for a "whole object" consisting of a large number of particles, as here gravity is not the only thing that does work on a single particle of that object, other particles does work on that particle as well. So how is "∆K = -∆Ugrav" true for a "whole object", when gravity does work only? Thank you for your time...