What force governs planetary motion and keeps planets in orbit around the Sun?

• A. Centrifugal force
• B. Inertia
• C. Gravity
• D. Electromagnetism

Answer: (C)

Gravity is the force that keeps planets in orbit around the Sun. The Sun’s gravity pulls on a planet, directing force toward the Sun. This inward pull provides the centripetal acceleration needed to bend the planet’s straight-line motion (inertia) into a closed curve. If the planet moves fast enough, gravity continually pulls it inward just enough to keep it from flying away, resulting in an orbit that can be a circle or an ellipse.

Centrifugal force isn’t the real cause here; it’s a fictitious effect that appears only when you describe motion from a rotating frame of reference. Inertia alone would carry the planet in a straight line unless something pulls it, and in the solar system that something is gravity. Electromagnetism isn’t the dominant interaction governing planetary orbits, which are governed by gravity between masses.