The time‐average forces exerted by the constraints of a vibrating system (radiation pressure, mean tension in a vibrating string, etc.) are easily computed from the adiabatic principle of Boltzmann and Ehrenfest (BE principle). The usual derivation from the “principle of varying action” is difficult.Here an equivalent principle (which gives in addition the time dependence of the forces) is derived from Lagrange's equations. It is also shown how similar results can be got from the solution of a linear differential equation with slightly variable coefficients.