This article addresses the calculation of the work done by inertia forces in the teaching of theoretical mechanics. It introduces an equivalent description of inertial forces based on the physics of reference frames. Based on this, the work done by inertia forces is re-examined. Firstly, Newtonian dynamics can be cleverly derived to yield an inevitable description that can be directly applied in a translational (non-rotating) reference frame by applying Newtonian dynamics to the reference object of the reference frame. Therefore, the theoretical expression for inertial forces is obtained through the real force acting on the reference object. Secondly, the traditional form of Newtonian dynamics can in principle be generalized to any translational reference frame and still holds by simply replacing the force terms from full to relative forces. Thus, it leads to a new approach for calculating the work done by the inertial force. Finally, as a check, the typical pedagogical topic of work done by inertial forces was re-examined and self-consistent results were obtained.