Hello, I have read over many textbooks and websites (including posts on this forum) relating to the concept of tension but I am still a bit confused. Many sources seems to only define tension rather than explain it. They seem to assume that you know it the way you know 1+1 =2. Here is the part where I am confused about. A rope can be seen as a coupling between railroad cars. Suppose I have a locomotive dragging a cart along with a coupling. The locomotive exerts a force of 100 N through the coupling to the cart. Now, by Newton's 3rd Law of motion, there must be a reaction force. The reaction force is where the cart is pulling back on the locomotive with the same force of 100 N but in the opposite direction. Now, the coupling have two forces pulling on it. Force 1: locomotive pulling on cart (100 N) Force 2: cart pulling on locomotive (100 N) Now, shouldn't the coupling be able to withstand 200 N because there are two forces at work here? All the reading I have done seems to suggest that the coupling only need to withstand 100 N but how could that be right? Doing so seems to ignore Newton's 3rd law. Thanks.