If a rod or wire of length L and cross-section area A under the action of stretching force F applied normally to its face suffers a increase ∆L in length, then Y=
tensile stress
longitudinal strain
=
F/A
∆L
L
Hence, Young's modulus is proportionality constant that relates the force per unit area applied perpendicularly at the surface of an object to the fractional change in length.