Cd'S

1) In elementary physics courses you are told that the resistance experienced by an oil drop of radius a falling with velocity v through air of viscosity T/ is given by Stokes's law: F = 6παηv. On the other hand, you may know that car makers always quote the air resistance in terms of a drag coefficient C_d of magnitude ~0.5. This gives the resisting force as F =½ C_dpAv² for a body of area A moving through air of density p. These two laws are not the same (as is clear from the different dependence on v).