The expression, (1+e )m_cv_c, groups together the three factors that contribute to a co-axial collision without external forces. They are:

1. Approach Velocity, embodied by the expression v_c.
2. Mass Effect, embodied by the expression m_c.
3. Structural Aspects, embodied by the expression (1+e ).

In effect, these three factors state that the intensity of the collision, i.e., the impulse generated by the impact, is governed solely by the velocity of approach between the objects, the effective mass of the objects, and the structural composition of the objects in the region on and behind the contact surfaces.

v_c is the velocity at which the two bodies approach each other. This factor points out that the intensity of the collision is not related to the individual velocity of either body. It is directly related to their relative velocity. In other words, a collision in which a stationary vehicle is struck in the rear by another advancing at 20 kilometres/hour, will have the same affect as one in which the first vehicle is driving forward at 10 kilometres/hour and the other is travelling in reverse at the same speed, or where the first is travelling at 120 kilometres/hour and strikes the second which is travelling at 100 kilometres/hour.

The first and third scenarios described above might be thought of as a low speed and a high speed collision respectively, and the overall consequences of the accident are likely to be quite different. But the danger from the high speed collision is due to the aftermath of the impact if the vehicles are out of control. The damage caused by the actual vehicle-to-vehicle contacts will be essentially identical.

m_c is called the effective mass. It is a more difficult concept to visualize. This factor shows that the intensity of the collision is not defined directly by either of the masses. Rather, the intensity is related to the combination of the masses expressed in the form of the m_c equation. The units of m_c are kg.

In co-axial theory, m_c is a valuable concept in that it can be used to analyze impacts varying from bodies of equal masses, to impacts where one body is deemed to be of infinite mass, e.g., test barriers, bridge abutments, etc. For the case where the masses are equal M = m and m_c = M/2 = m/2. If one of the masses equals infinity, i.e., m = µ , then m_c = (M ´ µ )/(M + µ ) @ M. Therefore we observe that m_c varies from half of the lighter mass when both masses are essentially equal, to the value of the lighter mass when the other is essentially infinite.

The structural factor deals with the generation of forces as the colliding surfaces deform. The expression (1+e ) is just one manifestation of the structural factor. Other manifestations deal with the shape of the two objects, and the extent to which energy is consumed in the form of crush.