Elastic Collision Relative Velocities

Steps:

• In a perfectly elastic collision, both total momentum and total kinetic energy are conserved.
• For one-dimensional collisions, the coefficient of restitution is 1, meaning relative speed of separation equals relative speed of approach.
• Option B states this property directly.
• Other options contradict conservation laws or add unrequired conditions.

Why B is correct:

• By definition, elastic collisions have coefficient of restitution e=1, so relative velocity after = - (relative velocity before), equating approach and separation speeds.

Why the others are wrong:

• A: Initial and final speeds swap only if masses are equal; otherwise, not true.
• C: Kinetic energy is conserved in elastic collisions, not reduced.
• D: Stationary instant occurs only if one starts at rest or specific conditions met, not always.

Final answer: B