Density

All right. Density, which is rho, is mass divided by volume. One thing to know is that for liquids, density is constant as they are incompressible. For gases, they're quite variable, variable density. If you look at the unit of the density in the SI, we're going to get kilogram per meter cube because kilogram is for the mass, and meter cube is for the volume. If you're looking at the British gravitational or English unit, you're going to get slug per feet cube.