I had an idea which i cant fully realize because i dont know the physics behind it. Here it is: imagine two particles, for example a and b. imagine also that a is attracted to b but b repels a. In this case, both atoms would essentially travel in a straight line forever assuming they repel and attract each other with the same force. what is wrong here? i know this cannot exist because it is a perpetual motion contradiction (there is no perpetual motion). But at the same time, they could go at a constant velocity, that is no acceleration. Can someone explain this to me in greater detail?