NAEP Science Grade 12

Category - Earth Science

Which of the following best summarizes Newton's first law of motion as it relates to the Moon's orbit around Earth? 
  1. The Moon's inertia keeps it moving tangent to its orbit, while Earth's gravitational field deflects it from a straight path.
  2. The Moon's rotation keeps it on the same path, while its velocity is maintained by Earth's gravity.
  3. The Moon's acceleration is driven by its mass, while its circular motion is maintained by its gravitational attraction to Earth.
  4. The Moon's weight causes it to fall toward Earth, while centripetal force accelerates it away from Earth.
Explanation
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).
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