# Inelastic Collisions between two objects

In an inelastic collision betweentwo objects, what exactly causes the system composed of the two objects to lose mechanical energy into heat?

I was trying to answer this question using the work energy theorem by setting the work done by the internal forces that both objects exert on one another to the change in kinetic energy of the two body system. I applied this to two different scenarios. In the first, object 2 is more massive than object 1. In the second, object 1 and object 2 have the same mass. Also, object 2 has an initial velocity of zero. If both masses are different, I believe that the work done by each internal force differs (the more massive object causes the least massive object to be displaced more in comparison to how much the more massive object is displaced by the least massive object). This is what I saw as what causes such a loss of kinetic energy. Is the answer to this question beyond introductory Newtonian mechanics? Is my method correct?