Automatically computing the reflected mass or reflected inertia of a computer-aided design model comprised of a motor includes executing a simulation of the model, using the simulation results to compute the reflected mass or reflected inertia, and treating the non-motor parts of the model as a virtual body having a time-varying mass or a time-varying inertia. The mass or inertia of the virtual body at a specific time is the reflected mass or reflected inertia, respectively, of the model at the specific time.