A wire of length L, Youngs modulus Y, and cross-sectional area A is stretched elastically by an amount L. By Hookes law (Section 7.4), the

A wire of length L, Young’s modulus Y, and cross-sectional area A is stretched elastically by an amount ΔL. By Hooke’s law (Section 7.4), the restoring force is ─ k ΔL.
(a) Show that k = YA/L.
(b) Show that the work done in stretching the wire by and amount ΔL is W = ½ YA (ΔL) 2/L