1. The problem statement, all variables and given/known data http://img131.imageshack.us/img131/2996/physicswb9.th.png [Broken] Two 100N weights are attached to a spring scale as shown. What does the scale read? 2. Relevant equations 3. The attempt at a solution This concept is difficult for me to grasp. My natural instinct would suggest that the scale would read 200N, but I know this can't be right. What I know (or think I know) is that the system is in static equilibrium with ΣF=0. The weight force of the block on the left is 100N, same for the block on the right. Now I notice that if I were to remove one of the blocks, say the left one, and tie the loose end to a wall, the solution becomes obvious, the scale would read 100N. I know the answer is 100N. What I don't know is why the answer is 100N. There is a fundamental concept of Newton's laws that I am not getting here. Specifically Newton's third law. Can someone please explain this to me as if I were a 2 year old, with big colourful pictures preferably drawn in crayon?