I am wondering about this: 1)Firtly, is it "reasonable" considering and studing the behaviour of two masses only, as if they were the only in the whole universe? 2) Suppose we can. Let as think we have two masses. A very big one, and a second times smaller than the first.These two masses are moving steadily against each other on the same line. Firtly we stand on the small mass and look towards the other. If we stand each time on a different mass, in both cases we want be able to tell the truth about the kinetic energy of the system. And if we try to do, we will conlclude in two different values for the kinetic energy. I think something must be wrong with one of the above statements or both of them.Has anyone a clue?