For a material of Young's modulus, Y (same as modulus of elasticity λ), cross sectional area, A 0, initial length, l 0, which is stretched by a length, Δ Δ l : U e = ∫ ∫ Y A 0 Δ Δ l l 0 d ( Δ Δ l ) = Y A 0 Δ Δ l 2 2 l 0 where U e is the elastic potential energy.