(S) descibres the shape elasticity of a material. Suppose that equal and opposite tangential forces F act on a rectangular block. These shearing forces distort the block, but its volume remains unchanged.

Shearing stress = tangential force acting / area of surface being sheared, or just simply σ s = F / A

Shearing strain = distance sheared / distance between surfaces, or just simply εs = ΔL / L0


S = F / A / [ ΔL / ΔL0 ] = FL0 / AΔL

Since ΔL is usually very small, the ratio ΔL / L0 is equal approximately to the shear angle Θ in radians. In that case

S = F / AΘ

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