The acceleration of a spacecraft is proportional to the vector sum of all the forces acting on the spacecraft. Each component of the resultant force is computed by individual force models. The required accuracy of force models is dependent on the magnitude of the force and the observability of the force in orbit determination software. By far the most important force model is gravity. Gravity force models are formulated as acceleration, but this is only a matter of convenience because the mass of the spacecraft factors out of the equations of motion.