1. The left end of a spring is attached to a wall, When Bob pulls on the right end with a 200N force, he stretches the spring my 20cm. The same spring is then used for a tug-of-war between Bob and Carlos. Each pulls on his end of the spring with a 200N force. How far does Bob's end of the spring move? 2. F=kx 3. Isn't this the same principal as the spring being attached to the wall. Doesn't the wall exert an equal and opposite 200N force since the spring does not move once it is strecthed? If this is so, I believe the answer would be 10cm, because Carlos pulls the spring another 10cm.