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When analyzing a two-dimensional collision where one particle starts at rest, what coordinate system choice does the source recommend?
AAn axis chosen at random for every new collision
BAn axis pointing straight up, regardless of either particle's motion
CAn axis parallel to the velocity of the incoming particle
DAn axis parallel to the final velocity of the particle that was at rest
Answer & Solution
Correct answer: C. An axis parallel to the velocity of the incoming particle
1. The source is choosing a coordinate system to make the algebra of a two-dimensional collision as simple as possible.
2. It recommends an axis parallel to the velocity of the incoming particle.
3. With that choice, the initially stationary particle contributes zero momentum along either axis before the collision, simplifying both conservation equations.
4. A vertical axis unrelated to either particle's motion, a random axis, or an axis tied to an outcome not yet known would not offer this same simplification.
_Source: OpenStax College Physics (CC BY 4.0), Ch 8 "Linear Momentum and Collisions", section 8.6 Collisions of Point Masses in Two Dimensions_
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