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11th Physics Topic - Hooke’s Law – Stress and Strain from Mechanical Properties of Solid  The ratio of the magnitude of the applied force to the cross-sectional area of the wire is called the stress acting on the wire, while the ratio of the change in length to the original length of the wire is called the strain of the wire. Above equation is a statement of Hooke’s law of elasticity, which says that in an elastic body the stress is directly proportional to the strain, that is,                                                         stress  ∝ strain The stress is what is applied to the body, while the resulting effect is called the strain. To make an equality out of this proportion, we must introduce a constant of proportionality. This constant depends on the type of material used, since the molecules, and hence the molecular forces of each material, are different. This constant, called Young’s modulus of elasticity is denoted by the letter Y. thus above equation becomes