A dashpot is a massless kinetic energy dissipating element used in iconic modeling for vibrations and mechanics. A dashpot acts a lot like a spring except instead of storing potential energy, dashpots dissipate kinetic energy. Like springs, dashpots exert a force except that the force is directed opposite to the direction of velocity, not motion. Again like springs, this force is linearly proportional to the magnitude of the velocity. The proportionality constant, denoted c, is typically called the damping constant and has units of N.s/m. Dashpots in series or parallel can be combined just like springs. Dashpots may also be torsional so that they exert a moment rather than a force. In this case the exerted moment is proportional to the angular velocity and oriented oppositely. Torsional dashpot constants are denoted ct and have units N.m.s/rad.
In Lagrange's equation the effect of dashpots are included in the energy dissipation term, ∆dashpot = (1/2)cv2.