(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 **ε**_{s} = Δ**L** / **L**_{0}

**Then...**

**S** = F / A / [ Δ**L** / Δ**L**_{0} ] = F**L**_{0} / AΔ**L**

Since Δ**L** is usually very small, the ratio Δ**L** / **L**_{0} is equal approximately to the shear angle **Θ** in radians. In that case

**S** = F / A**Θ**