Question

The earth moves in an elliptical orbit with the sun S at one of foci as shown in the figure. Its rotational kinetic energy is maximum at the point

• A. A
• B. B
• C. C
• D. D

Answer 1

Answer: (A)

The earth revolves around the sun once in a year. In order to find the rotational kinetic energy about the sun, we may treat the earth as a point mass, of moment of inertia I. If $\omega$ is angular velocity of earth, then
$K_{rot}=\frac{1}{2}I{\omega }^2$
In planetary motion, angular momentum remains conserved.
At A, the moment of inertia is least, so angular speed is maximum. So, at A, rotational kinetic energy is maximum.