Hooke's Law is used to describe the behavior of springs and other elastic objects when stretched by a force. It states that the amount of deformation is in a linear relationship with the force used, so that twice a given force produces twice the deformation. Hooke's Law applies only to stress forces, so at high levels of force, this relationship breaks down.
Hooke's Law is the law of elasticity first discovered by Robert Hooke in 1660. Every spring or other elastic object obeys this law, but each varies in its level of resistance to stretching. Objects with more resistance stretch less under the same forces, but no matter the resistance, doubling the force doubles the stretch. This is only true up to a point, however.
To understand Hooke's Law and its valid range of forces, the difference between stress and strain must be understood. Stress is the amount of force experienced in each unit portion of an object. For instance, pressure is often described in pounds per square inch. This is a measure of stress. Strain, on the other hand, is the amount of deformation of each unit under stress. At relatively low levels, stress and strain are proportional, and it is only in this range that Hooke's Law is valid.