Which statement correctly describes the forces between the Moon and Apollo 11 during the lunar landing?

• A. The Moon pulls on Apollo 11 with the same force that Apollo 11 pulls on the Moon, in opposite directions.
• B. The Moon exerts a greater gravitational force on Apollo 11 than Apollo 11 exerts on the Moon.
• C. Only the Moon exerts gravitational force on Apollo 11.
• D. Apollo 11 pulls on the Moon with a greater force than the Moon pulls on Apollo 11.

Answer: (B)

When two bodies interact gravitationally, they exert forces on each other that are equal in strength and opposite in direction. This is Newton’s third law: the action-reaction pair. The Moon pulls on Apollo 11 with a certain force, and Apollo 11 pulls on the Moon with the same force in the opposite direction. The size of that force comes from the gravitational formula F = G m1 m2 / r^2, which depends on both masses and their distance, but swapping which body is “the source” does not change the magnitude of the force. So the forces are mutual and equal, just directed along the line joining the two bodies. The idea that one body exerts a greater pull, or that only one of the bodies exerts gravity, doesn’t fit how gravity works between two objects.