The engineering stress (by definition) is the first derivative of the energy in terms of the extension ratio, which is equivalent to the concept of strain: σ σ eng = d ( Δ Δ f def ) λ λ z = k B T v s β β ( λ λ z − − 1 λ λ z 2 ) and the Young's Modulus E is defined as derivative of the stress with respect to strain, which measures the stiffness of the rubber in laboratory experiments.