Elastic magnetic interaction conserves kinetic energy

Steps:

• Magnets P and Q approach with relative speed $v_\text{approach}$.
• They interact magnetically without contact, repelling each other.
• After interaction, relative speed of separation $v_\text{separation} = v_\text{approach}$.
• Equal relative speeds indicate elastic interaction, conserving total kinetic energy.

Why D is correct:

• In elastic collisions, total kinetic energy is conserved by definition, so KE of P + KE of Q before equals that after.

Why the others are wrong:

• A: Kinetic energy is conserved, not lost.
• B: Total momentum is conserved due to no external forces on the frictionless track.
• C: Momenta before need not be equal; depends on masses and individual speeds, only relative speed is given.

Final answer: D