The term drift arose in Section 13.2.1 and implies that from some states it is impossible to instantaneously stop. This difficulty arises in mechanical systems due to momentum. Infinite deceleration, and therefore infinite energy, would be required to remove all kinetic energy from a mechanical system in an instant of time. Kinodynamic and trajectory planning generally involve drift. Nonholonomic planning problems may be driftless if only velocity constraints exist on the C-space; the models of Section 13.1.2 are driftless. From a planning perspective, systems with drift are usually more challenging than driftless systems.