Hi - When an engine uses chemical energy to move a vehicle, the fuel loses mass in the conversion, so that momentum (mv) stays the same before and after the acceleration? If this is true, then we convert mass to velocity all the time, while keeping mv constant, right? Where does velocity become mass? Or is all mass becoming velocity, and everything eventually ends up with a lot of velocity and no mass? You can see where I need clarification I'm sure. Thank you for your time.