Hooke’s law states that the restoring force exerted by an ideal spring is directly proportional to its displacement from equilibrium, expressed as F = -kx, where k is the spring’s stiffness constant. It is the foundational physical relationship behind compliant and series elastic actuation in robotics, where a spring element is deliberately placed in the drivetrain so that measuring its deflection yields an accurate estimate of applied force or torque. The law holds only within a spring’s elastic limit, beyond which deformation becomes non-linear or permanent.