Conservation of mass: Mass, like energy, is a conserved property, and it cannot be created or destroyed during a process. Closed systems: The mass of the system remain constant during a process. Control volumes: Mass can cross the boundaries, and so we must keep track of the amount of mass entering and leaving the control volume.
Mass m and energy E can be converted to each other according to
where c is the speed of light in a vacuum, which is c = 2.9979 × 108 m/s. The mass change due to energy change is negligible.