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: normal forces and shear forces. A normal force acts normally over an area and a shear force acts tangentially over an area.

Block on a ramp and corresponding free body diagram of the block showing the surface force from the ramp onto the bottom of the block and separated into two components, a normal force N and a frictional shear force f, along with the body force of gravity mg acting at the center of mass.

Equations for surface force

Surface force due to pressure

${\displaystyle f_{s}=p\cdot A\ }$, where f = force, p = pressure, and A = area on which a uniform pressure acts

Examples

Pressure related surface force

Since pressure is ${\displaystyle {\frac {force}{area}}=\mathrm {\frac {N}{m^{2}}} }$, and area is a ${\displaystyle (length)\cdot (width)=\mathrm {m\cdot m} =\mathrm {m^{2}} }$,

a pressure of ${\displaystyle 5\ \mathrm {\frac {N}{m^{2}}} =5\ \mathrm {Pa} }$ over an area of ${\displaystyle 20\ \mathrm {m^{2}} }$ will produce a surface force of ${\displaystyle (5\ \mathrm {Pa} )\cdot (20\ \mathrm {m^{2}} )=100\ \mathrm {N} }$.

See also

• Body force
• Contact force