UPSC CDS I 2019 GK Paper

Concept:Hooke’s law applies only to the initial linear portion of the stress‑strain curve, called the proportional region.Explanation:The law states that stress is directly proportional to strain, i.e., $\text{Stress} \propto \text{Strain}$, so $\frac{\text{Stress}}{\text{Strain}} = \text{Young’s modulus (constant)}$.This proportionality holds only until the proportional limit, which is within the elastic region but not the whole elastic region.Beyond the proportional limit (even within the elastic limit), the relationship becomes non‑linear and Hooke’s law is no longer valid.Thus, the law is restricted to the proportional region of the stress‑strain curve.Answer:A. only proportional region of the stress‑strain curve