Newton's Third Law Action-Reaction Pairs

Steps:

• Identify Newton's third law: Forces between two interacting objects are equal in magnitude and opposite in direction.
• Examine P: Force on Moon due to Earth (Earth-Moon interaction).
• Examine R: Force on Earth due to Moon (same Earth-Moon interaction).
• Confirm P and R form the action-reaction pair; other pairs involve different objects.

Why B is correct:

• P and R are the mutual gravitational forces between Earth and Moon, equal and opposite per Newton's third law.

Why the others are wrong:

• A: P (Earth-Moon) and Q (Sun-Earth) involve different object pairs.
• C: Q (Sun-Earth) and S (Sun-Moon) act on different objects from the Sun.
• D: R (Earth-Moon) and S (Sun-Moon) involve different interacting pairs.

Final answer: B