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When one car bumps into another from behind and both continue moving, the source uses Newton's third law to show something about their combined momentum. What is it?
AOnly the trailing car experiences any change in momentum
BEach car's own momentum stays constant throughout the collision
CThe total momentum of the two-car system doubles during the collision
DThe two changes in momentum are equal and opposite, keeping total momentum constant
Answer & Solution
Correct answer: D. The two changes in momentum are equal and opposite, keeping total momentum constant
1. By Newton's third law, the force car 2 exerts on car 1 is equal and opposite to the force car 1 exerts on car 2.
2. Since both forces act for the same collision time, the resulting changes in momentum for the two cars are also equal and opposite.
3. This leads to the two changes in momentum adding to zero, so the two-car system's total momentum is conserved.
4. Individually, car 1 loses momentum and car 2 gains momentum, so neither car's own momentum stays fixed on its own.
_Source: OpenStax College Physics (CC BY 4.0), Ch 8 "Linear Momentum and Collisions", section 8.3 Conservation of Momentum_
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