Question

which of the following best describes keplers third law of planetary motion? a the gravitational force between two objects is inversely proportional to the square of the distance between them. b the speed of a planet is fastest when it is closest to the sun. c the planets move in elliptical orbits with the sun at one focus. d the square of the orbital period of a planet is directly proportional to the cube of the semi - major axis of its orbit.

Explanation:

Kepler's Third Law states that the square of the orbital period ($T$) of a planet is directly proportional to the cube of the semi - major axis ($a$) of its orbit, expressed as $T^{2}\propto a^{3}$. Option a describes Newton's law of universal gravitation. Option b is Kepler's Second Law (law of equal areas). Option c is Kepler's First Law.

Answer:

D. The square of the orbital period of a planet is directly proportional to the cube of the semi - major axis of its orbit.