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

The Correct Option is A

In planetary motion, angular momentum remains conserved. 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 $K_{\text {rot }}=\frac{1}{2} I \omega^{2}$ 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.