Suppose there are two objects, denoted by A and B. When they collide with each other with the condition that A is stationary and B is in motion, B exerts an action force on A due to which A exerts a reaction force on B. Thus the net force exerted on A is $F_{{action}_{AB}}$ and the net force exerted on B is $F_{{reaction}_{BA}}$. Same is the case when A is in motion and B is at rest.
But when A and B collide when they are in motion, the net force exerted on A is $F_{{action}_{AB}}$ + $F_{{reaction}_{AB}}$ and the net force exerted on B is $F_{{action}_{BA}}$ + $F_{{reaction}_{BA}}$.
Question: Is my reasoning correct for the case when A and B collide when they are in motion? Is the net force exerted on A and B when they collide when they are in motion greater in magnitude than the net force exerted on A and B when they collide when only one of them is in motion?
Best Answer
If I understand correctly your reasoning, you are using incorrectly the Newton's third law. The force of A is not exerted over A itself, and the same for B.