If we have a perfect gas of mass density ρm kg/m3, and we assign a velocity U m/s to each...

If we have a perfect gas of mass density ρm kg/m3, and we assign a velocity U m/s to each differential element, then the mass flow rate is ρmU kg/(m2 – s). Physical reasoning then leads to the continuity equation,  (a) Explain in words the physical interpretation of this equation. (b) Show that  where M is the total mass of the gas within the constant closed surface S, and explain the physical significance of the equation.