Mass is the product of density and volume. Mass flow rate is the product density, velocity, and cross-sectional area. (It's the derivative of mass with respect to time.) Bare with the syntax please... Looking at a sphere within a larger sphere, the volume of the difference is V=(4/3)*pi*(R^3-r^3) where R is a constant inner radius of the larger sphere. r is the radius of the smaller sphere, and it's not constant. Multiplying that by density gives us the mass of the region in between the spheres. Taking the derivative of that mass, however, with respect time, how would the equation look in relation to the description I gave above in the second line? This isn't homework as I'm not in school anymore. It's a personal question. It's a situation where the smaller sphere is increasing in radius as time is increased.