Final answer:
When two spring scales push against each other with the same force, they exert equal and opposite forces, so neither moves. In a collision on a frictionless surface, the center-of-mass velocity remains the same because momentum is conserved.
Step-by-step explanation:
The original question asks what happens when spring scale A and spring scale B both exert a 4 N force on each other. When two spring scales push against each other with the same force, they essentially exert equal and opposite forces on each other. According to Newton's third law, these forces will cancel each other out, so neither scale will move as a result of these forces. This concept is related to the conservation of momentum and the principle of action-reaction pairs.
To calculate the center-of-mass velocity both before and after the collision of two masses on a frictionless surface, one would use the formula for the conservation of momentum, which states that the total momentum before the collision is equal to the total momentum after the collision. The calculation would show that the center-of-mass velocity remains unchanged by the collision, as momentum is conserved in an isolated system.