Question: Spheres of various diameters D and densities σ are allowed to fall freely under gravity through various fluids (represented by their densities ρ and kinematic viscosities ν) and their terminal velocities V are measured. Find a rational expression connecting V with the other variables, and hence suggest a suitable graph in which the results can be presented. Note: There will be five unknown indices, and therefore two must remain undetermined, which will give two unknown functions on the right-hand side. Make the unknown indices those of σ and ν.