Question

A planet is moving round the sun in an elliptical orbit as shown. As the planet moves from A to B

• A. its kinetic energy will decrease
• B. its potential energy will remain unchanged
• C. its angular momentum about centre ofsun will remain unchanged
• D. its speed is minimum at A

Answer 1

Answer: (C), (D)

The correct options are
C its angular momentum about centre ofsun will remain unchanged
D its speed is minimum at A

Angular momentum of the planet about the centre of the sun is constant because the gravitational force of attraction passes through the centre of the sun. Since at A, planet is farthest from sun, therefore, its speed will be minimum at A. On moving from A to B, the distance between the planet and the sun decreases therefore, speed increases and hence kinetic energy increases. Since distance decreases, potential energy also decreases.

So correct options are C and D.