Surface force

Surface force denoted f_s is the force that acts across an internal or external surface element in a material body. Surface force can be decomposed into two perpendicular components: pressure and stress forces. Pressure force acts normally over an area and stress force acts tangentially over an area.

f = Force, p = pressure, A = cross sectional area of the moving fluid

Pressure is in ${\displaystyle {\frac {force}{area}}=\mathrm {\frac {N}{m^{2}}} }$
Area is a ${\displaystyle (length)\cdot (width)=\mathrm {m\cdot m} =\mathrm {m^{2}} }$

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