Surface forces act on walls of the cubic fluid element. For each wall, the X component of these forces was marked in the figure with a cubic element (in the form of a product of stress and surface area e.g. − − σ σ x x d y d z with units P a ⋅ ⋅ m ⋅ ⋅ m = N m 2 ⋅ ⋅ m 2 = N ).