Elastic collision between equal masses
A billiard ball moving at 2 m/s hits an identical ball at rest head-on in a perfectly elastic collision. What happens?
Answer A: explanation for every choice
A
Correct. Conserving both momentum and kinetic energy with equal masses requires the velocities to be exchanged.
B
Moving together describes a perfectly inelastic collision, which loses kinetic energy.
C
This would violate conservation of momentum.
D
This would double both momentum and kinetic energy.
TakeawayEqual masses, head-on, elastic: velocities swap.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID cp61.