 In the case of a perfect fluid this component is

where formula_4 is the relativistic mass per unit volume, and for an electromagnetic field in otherwise empty space this component is

where "E" and "B" are the electric and magnetic fields, respectively.The flux of relativistic mass across the "xi" surface is equivalent to the density of the "i"th component of linear momentum,

The components
represent flux of "i"th component of linear momentum across the "xk" surface.In particular,
(not summed) represents normal stress, which is called pressure when it is independent of direction.