Gravitational Force Enables Orbital Motion

Steps:

• Planets follow curved paths around the Sun, requiring a centripetal force directed inward.
• This force must act toward the center of the orbit to maintain the circular or elliptical path.
• The only force acting between the Sun and planets is gravity, which is always attractive.
• Thus, the gravitational force pulls each planet toward the Sun.

Why D is correct:

• Newton's law of universal gravitation states that the force between two masses is attractive and directed along the line joining their centers, pulling planets toward the Sun.

Why the others are wrong:

• A: Gravity is attractive, not repulsive, so no force pushes planets away.
• B: No force opposes the planet's tangential velocity in ideal orbits; that would slow it down.
• C: A force in the direction of travel would speed up the planet linearly, not cause orbiting.

Final answer: D