Question

Which of the following quantities remain constant in a planetary motion (consider elliptical orbits) as seen from the sun?
(a) Speed
(b) Angular speed
(c) Kinetic Energy
(d) Angular momentum.

Answer 1

Answer:

kinetic energy

Explanation:

because it is a form of energy which remains constant

Answer 2

Angular momentum remains constant in a planetary motion as seen from the sun.

Explanation:

• As the net external torque on the planet is zero in an elliptical orbit motion, the angular momentum remains constant.  Speed in an elliptical orbit depends on the radius which varies at various points.  
• Hence, the velocity in an elliptical orbit is not same throughout the orbit.  As the velocity is not same, the kinetic energy varies throughout the orbit.
• In addition, angular speed is not constant as the angle varies in an elliptical orbit.