Which of the following quantities is conserved for a planet orbiting a star in a circular orbit? Only the planet itself is to be taken as the system; the star is not included.
A) Momentum and energy.
B) Energy and angular momentum.
C) Momentum and angular momentum.
D) Momentum, angular momentum and energy.
E) None of the above.

Question

Quizplus · Accepted Answer

For a planet orbiting a star in a circular orbit, the only force acting on the planet is the gravitational force, which is a conservative force. Therefore, the mechanical energy of the planet (the sum of kinetic and potential energy) remains constant throughout the orbit. Additionally, since the orbit is circular, the distance between the planet and the star does not change, and so the angular momentum of the planet about the center of the orbit (which is the center of the star) remains constant as well. Therefore, energy and angular momentum are conserved quantities for a planet in a circular orbit around a star, but momentum is not, as the planet is constantly changing direction.

Which of the Following Quantities Is Conserved for a Planet