In the elementary form of the continuity equation for mass, in hydrodynamics:
In elementary classical mechanics, mass flow rate is encountered when dealing with objects of variable mass, such as a rocket ejecting spent fuel. Often, descriptions of such objects erroneously invoke Newtons second law  by treating both the mass  and the velocity  as time-dependent and then applying the derivative product rule. A correct description of such an object requires the application of Newtons second law to the entire, constant-mass system consisting of both the object and its ejected mass.
Mass flow rate can be used to calculate the energy flow rate of a fluid:
where  is the unit mass energy of a system.
Energy flow rate has SI units of kilojoule per second or kilowatt.