Q85 / 90 · KVPY 2012 (SB/SX) · Physics
A planet orbits in an elliptical path of eccentricity e around a massive star considered fixed at one of the foci. The point in space where it is closest to the star is denoted by P and the point where it is farthest is denoted by A. Let $\mathrm{v}_{\mathrm{p}}$ and $\mathrm{v}_{\mathrm{a}}$ be the respective speeds at $\mathrm{P}$ and $\mathrm{A}$. Then-


- Correct answerA$\frac{\mathrm{v}_{\mathrm{P}}}{\mathrm{v}_{\mathrm{A}}}=\frac{1+\mathrm{e}}{1-\mathrm{e}}$
- B$\frac{v_{p}}{v_{A}}=\frac{1+e^{2}}{1-e}$
- C$\frac{\mathrm{v}_{\mathrm{P}}}{\mathrm{v}_{\mathrm{A}}}=1$
- D$\frac{v_{P}}{v_{A}}=\frac{1+e^{2}}{1-e^{2}}$
Solution
Using conservation of angular momentum
$\begin{array}{l}
\mathrm{V}_{\mathrm{p}} \cdot r_{\mathrm{p}}=\mathrm{V}_{\mathrm{A}} \cdot \mathrm{r}_{\mathrm{A}} \\
\frac{\mathrm{V}_{\mathrm{P}}}{\mathrm{V}_{\mathrm{A}}}=\frac{\mathrm{r}_{\mathrm{A}}}{\mathrm{r}_{\mathrm{P}}}=\frac{\mathrm{a}+\mathrm{ae}}{\mathrm{a}-\mathrm{ae}}
\end{array}$
Hence, the correct answer is option: 'A'.