Properties of elastic collisions

Steps:

• Recall that a perfectly elastic collision conserves both momentum and total kinetic energy.
• The coefficient of restitution e = 1 for elastic collisions, meaning relative speed of separation equals relative speed of approach.
• Option A matches both conditions exactly.
• Eliminate options that violate either conservation law.

Why A is correct:

• By definition, elastic collisions conserve kinetic energy and have e=1, so relative speed of approach equals separation (from the formula: e = (v2' - v1') / (u1 - u2)).

Why the others are wrong:

• B: Relative speeds must be equal in elastic collisions, per e=1.
• C: Kinetic energy is conserved in elastic collisions, not lost.
• D: Kinetic energy conservation is required for elastic collisions.

Final answer: A