Final answer:
Isaac Newton's law of universal gravitation combined with his first law of motion explains how gravitational forces and inertia collaborate to keep planets in orbit around the sun, with gravitation accounting for the planetary motion.
Step-by-step explanation:
Isaac Newton's law of universal gravitation and his first law of motion collectively explain how planets stay in orbit around the sun. Newton's law of universal gravitation states that every object in the universe attracts every other object with a gravitational force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This law, when applied to planetary motion, describes the attractive force that the sun exerts on the planets, causing them to orbit. Newton's first law of motion, often referred to as the law of inertia, states that an object will continue in its state of rest or uniform motion in a straight line unless acted upon by an external force. In the context of planetary orbits, this means that a planet would continue to travel in a straight line if not for the gravitational pull of the sun, which compels it to follow a curved path or orbit.
The correct answer to the question, 'How do Isaac Newton's law of universal gravitation and his first law of motion help us understand how the planets stay in orbit?' is C. Newton's law of universal gravitation accounts for planetary motion, and not merely Newton's first law by itself. It is the combination of gravitational force and the law of inertia that keeps the planets in their elliptical orbits, according to Newton's synthesis of celestial and terrestrial mechanics.