Figure 1. Lever–load systems. (a) Torque is the product of the magnitude of a force, F, and the perpendicular distance, r, between the line of action of the force and the centre of rotation (COR). (b) A first-order lever–load system; (c) a second-order lever–load system; (d) a third-order lever–load system.

Figure 2. Principal stresses and strains that develop in response to (a) axial loading, (b) shear and (c) torsion. Axial loading causes tension (₁) and compression (₂) at orientations perpendicular to each other. Shearing leads to parallel but noncollinear forces in the same plane, computed as x/y or tan . Torsion occurs when an element is twisted around an axis, generating tension, compression and shearing stresses. In torsion, cross-sectional stresses, , that occur in response to an applied torque, T, are proportional to Tr/J where r is the radius and J is the polar moment of inertia (the sum of I_x + I_y).

Figure 3. (a) A load–deformation curve, which characterizes the mechanical property of brittle, stiff and compliant materials. Y is the yield point, after which plastic deformation occurs; F is the fracture point, at which the material breaks. The Young modulus of elasticity is the slope of the curve (y/x). (b) Stress–strain curve for an elastic material during loading and unloading. The shaded area represents the energy lost (usually as heat).