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- Figure

  • Correct answer
    A
    $\frac{\mathrm{v}_{\mathrm{P}}}{\mathrm{v}_{\mathrm{A}}}=\frac{1+\mathrm{e}}{1-\mathrm{e}}$
  • 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'.

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