The mechanical analog of a capacitor is the compliance of a spring. The voltage across a capacitor corresponds to the force used to displace a spring. The charge stored in the capacitor corresponds to the displacement of the spring. Thus, Eq. (E.2) corresponds to Hooke's law for ideal springs:
where is called the spring constant or spring stiffness. Note that Hooke's law is usually written as . The quantity is called the spring compliance.