A force that acts parallel to a plane rather than perpendicularly, as with a tensile or compressive force. A shear stress requires a combination of four forces acting over (most simply) four sides of a plane and produces two equal and opposite couples. It is measured as the ratio of one shearing force to the area over which it acts, F/(ab) in the diagram. The shear strain is the angular deformation, θ‎, in circular measure. The shear modulus is the ratio of the shear stress to the shear strain (see also elastic modulus).

Shearing force.