Mechanical properties of solids
A change of a body’s shape or size under an external force is called deformation. If the body returns to its original state when the force is removed, the deformation is elastic; if not, it is plastic. The absolute elongation is Δl = l – l₀, the relative elongation ε = Δl/l₀ (dimensionless, often in per cent). Mechanical stress σ = F/S is the elastic force per unit cross-section area, unit Pa. For small elastic deformation Hooke’s law holds: σ = E|ε|, where E is Young’s modulus, a property of the material (about 2·10¹¹ Pa for steel, about 7·10¹⁰ Pa for aluminium). If the stress exceeds the strength limit the body breaks; brittle materials (glass, cast iron) break with almost no plastic deformation, while ductile ones (copper, lead) first stretch. Hooke’s law can also be written F = ES·Δl/l₀, which matches the spring law F = kΔl with k = ES/l₀.
Experiment (with an adult): mark a long rubber band or soft rubber cord and hang increasing loads on it one by one, measuring Δl. After removing the load, does the band return to its original length? When does the deformation become plastic? Do not hang heavy loads above your feet; work away from where a load could fall.