The classical equations of motion may be written in the following form: 
where N is the number of degrees of freedom;
are the vectors of generalized positions, velocities, and accelerations, respectively;
is the system mass matrix;
is the vector of Coriolis and centrifugal forces;
is the vector of gravitational forces;
and 
is the vector of generalized forces.
The motion of the model is completely defined by the generalized positions, velocities, and accelerations. Consequently, all of the terms on the left-hand side of the equations of motion are known. The remaining term on the right-hand side of the equations of motion is unknown. The inverse dynamics tool uses the known motion of the model to solve the equations of motion for the unknown generalized forces.