The forces that act on car 1 and car 2 must come from within the system. Option C
What is the principle of conservation of momentum?
The principle of conservation of momentum states that the momentum before collision must be the same as the momentum after collision. In other words, the total momentum in the system must be a constant.
As we can see, the system is closed in such a way that external forces do not act on the cars and any of the forces that acts must come from within the system and is constant. This is in consonance with the law of conservation of linear momentum.
Having known that in a system were the momentum is conserved, there are no external forces that act on the system. We can then conclude that the forces that act on car 1 and car 2 must come from within the system.
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