I know that if gravity is the only thing that does work on a "particle"particle, theits change of kinetic energy of that "particle" energy will be equal to the work done by gravity(∆K = -∆Ugrav). I know that "∆K = -∆Ugrav" is true for a "particle". But $$\Delta K = -\Delta U_{\rm grav}. \tag{1}\label{energyCons}$$ But how is that true for a "whole object"an object consisting of a large number of particles, as here? There gravity is not the only thing that does work on a single particle of thatthe object, other particles do work on that particle as well. So how is "∆K = -∆Ugrav"Eq. \eqref{energyCons} true for a "whole object", when only gravity does work only? Thank you for your time...