I had a question that was on my mind for a while. The general question is about structures and a force on that object in a certain direction and how the entire structure is accelerated by this force. An example: If I act a force on two trains with equal mass that are attached to each other, both trains acceleration will be the same. What forces act on the suspension point of the trains? I know the total force can't be zero, because the suspension point is also accelerating. This is just a small example, but in high school I only got examples of how forces that acted on whole objects and not the effect on multiple objects. Thanks, vaart BTW, this is my first post on PF, so please let me know if I didn't do something right. And offcourse: thanks for you help in advance!