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A satellite revolves around a planet in a circular orbit of radius R with the time period \(t\) . If the orbital radius is 3R, then its time period of revolution is

  • A.\(t\)
  • B.\(2t\)
  • C.\(\sqrt{27}t\)
  • D.\(\sqrt{3}t\)
  • E.\(2\sqrt{2}t\)

Answer: C

The new period is \(\sqrt{27}t\).

By Kepler's third law \(T^{2} \propto R^{3}\), so \(T \propto R^{3/2}\):

\[ \begin{aligned} \frac{T'}{t} &= \left(\frac{3R}{R}\right)^{3/2} \\ &= 3^{3/2} \\ &= \sqrt{27} \end{aligned} \]

So \(T' = \sqrt{27}t = 3\sqrt{3}t\), about 5.2 times longer. \(\sqrt{3}t\) uses \(T \propto R^{1/2}\), and \(t\) would mean the period is independent of the radius.

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