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Stress is the internal restoring force per unit area developed when a body is subjected to a deforming force: sigma = $\frac{F}{A}$. Its SI unit is Pascal (Pa = N $m^{-2}$) and dimensional formula is [M $L^{-1}$ $T^{-2}$].

Four types of stress arise from different loading conditions. Tensile stress occurs when forces pull the body along its length (stretching a wire). Compressive stress develops when forces push inward (pillars supporting a building). Shear stress (tau = $\frac{F}{A}$) results from tangential forces that deform the shape without changing volume (pushing the top of a book while the bottom is fixed). Volumetric (hydraulic) stress equals the change in pressure Delta P when uniform pressure acts from all sides (a ball submerged in water).

Key points for problem-solving: (1) Stress is force per unit area, so doubling the diameter quadruples the area and quarters the stress. (2) Breaking stress is a material property — independent of dimensions. Breaking force = breaking stress times area, so it depends on cross-section. (3) Normal stress acts perpendicular to the surface; shear stress acts parallel. (4) In a wire or rod, the stress is uniform across the cross-section only for axial loading. For bending, stress varies linearly from the neutral axis.