Relative Motion, Principle of

(Maxwell’s theory), states that for a linearly deformed body the displacement 8_ki of a point of application of a single force P_k of the first state (see Figure 1, a) in the direction of its action caused by any other single force P₁ of the second state (see Figure 1, b) is equal to the displacement δ_ik of the point of application of the force P_i in the line of action of single force Pk—that is, δik = δ_ki

Figure 1. Displacements (deflections) of a simple beam under the influence of single forces: (a) first state, (b) second state

The principle of relative motion, first formulated by the English physicist J. Maxwell, is a special case of the principle of relative work; it is widely used in strength of materials and in structural mechanics in the calculation of elastic systems.