Correct Response: A. Newton's first law of motion, the Law of Inertia, states that an object either remains at rest or continues to move at a constant speed in the same direction, unless acted upon by an external force. Earth's gravitational field exerts an external force on the Moon, pulling it toward Earth. This force deflects the Moon from moving in a straight line tangent to the curved path of its orbit. Although the Moon's orbital path, velocity, and rotation (B) are affected by its inertia and rotational inertia, the interaction of these factors does not summarize Newton's first law. The Moon does accelerate as it falls toward Earth during its orbit (C), but the gravitational interaction that causes the acceleration does not summarize Newton's first law any more effectively than do the concepts of weight and centripetal force (D).
