When talking about collisions, we usually talk about momentum because it allows us to analyze the situation in an easy way, since it is conserved. However, forget for a moment about the momentum. I had a confusion about Newton's Third Law in the situation of a collision.
Consider the following scenario: in a 1D situation, a mass 'm' at the left moves with a velocity '+v', such that it collides with an equal mass 'm' at the right which moves with a velocity '-v'. At the moment of impact, how are the forces in this situation?
I imagine two possible answers, but I don't know which one is the correct answer.
- The mass at the left pushes the mass at the right with a force F, and the mass at the right pushes the mass at the left with a force -F. Therefore, both masses feel a force |F|.
- The mass at the left pushes the mass at the right with a force F, and this causes a reaction, pushing the mass at the left with a force -F. Further, the mass at the right will also exert another push of force -F to the left, and this force causes a reaction, pushing the mass at the right with a force F. Therefore, both masses feel a force |2F|.
Which answer is correct and why? It seems like the first answer is correct because the force of one causes a force to that mass due to the other mass. But, the second possible answer would also be reasonable, because since the second mass was also moving, it will also cause a force on the first mass, which causes a force on the second mass due to the first mass.
Best Answer
The first scenario is correct. Remember that a force must have a cause, a source. In the first scenario, when object A hits object B, then A causes the force on B and B causes the force on A.
In the second scenario you can't identify the source, it seems. It seems that the force of A on B also "causes" the force on A itself. A force doesn't cause a force. This scenario seems off. Also, why stop there? Why wouldn't the second force, produced by the first force, produce a third force? Which in turn will produce a fourth force and so on? This thought experiment seems to have no limiting principle and would thus result in infinite force. Which is obviously not what we see in reality - so this scenario must be based on an incorrect assumption.