If the rest mass is constant the equation of motion may be presented in the form: force equals time rate of change in momentum. However, if the rest mass varies because there is an influx or or efflux of mass, the equation of motion may include additional terms to account for the momentum flux. These terms are derived by applying the impulse-momentum principal and the work-energy principle. The relativistic equation of motion gives the thrust as a function of the influx or efflux velocity, the rate of mass flow, and the Lorentz factor[1 − (v2/c2)]12for the system.