text: Ram pressure is a pressure exerted on a body moving through a fluid medium, caused by relative bulk motion of the fluid rather than random thermal motion. It causes a drag force to be exerted on the body. Ram pressure is given in tensor form as
  where  is the density of the fluid;   is the momentum flux per second in the  direction through a surface with normal in the  direction.  are the components of the fluid velocity in these directions. The total Cauchy stress tensor  is the sum of this ram pressure and the isotropic thermal pressure (in the absence of viscosity).
  In the simple case when the relative velocity is normal to the surface, and momentum is fully transferred to the object, the ram pressure becomes