Elastic force

A change in a body’s shape or size under an external force is called deformation. Types: stretching, compression, bending, shear and torsion. If the body returns to its former shape when the force is removed the deformation is elastic; if not, it is plastic. A deformed body develops an elastic force that tends to restore its former state; it is directed against the deformation. Hooke’s law: for small elastic deformations the magnitude of the elastic force is directly proportional to the elongation (compression): F = kΔl, where k is the stiffness, unit N/m, and Δl is the elongation in metres. Stiffness depends on the material, size and shape of the body. The support reaction holding a book on a table and the tension of a string holding a hanging load are also elastic forces.