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JEE Advanced · Physics · 9. Gravitation

A thin uniform annular disc (see figure) of mass \(M\) has outer radius \(4 R\) and inner radius \(3 R\). The work required to take a unit mass from point \(P\) on its axis to infinity is

  1. A \(\frac{2 G M}{7 R}(4 \sqrt{2}-5)\)
  2. B \(-\frac{2 G M}{7 R}(4 \sqrt{2}-5)\)
  3. C \(\frac{G M}{4 R}\)
  4. D \(\frac{2 G M}{5 R}(\sqrt{2}-1)\)
Verified Solution

Answer & Solution

Correct Answer

(A) \(\frac{2 G M}{7 R}(4 \sqrt{2}-5)\)

Step-by-step Solution

Detailed explanation

\(W=\Delta U =U_f-U_i=U_{\infty}-U_p \)
\( =-U_P \)
\( =-m V_P \)
\( =-V_P \quad(\text { as } m=1)\)
Potential at point \(P\) will be obtained by integration as given below

Let \(d M\) be the mass of small ring as shown
\(d M =\frac{M}{\pi(4 R)^2-\pi(3 R)^2}(2 \pi r) d r \)
\( =\frac{2 M r d r}{7 R^2} \)
\( d V_P =-\frac{G \cdot d M}{\sqrt{16 R^2+r^2}} \)
\( =-\frac{2 G M}{7 R^2} \int_{3 R}^{4 R} \frac{r}{\sqrt{16 R^2+r^2}} \cdot d r \)
\( =-\frac{2 G M}{7 R}(4 \sqrt{2}-5) \)
\( \therefore \quad W =+\frac{2 G M}{7 R}(4 \sqrt{2}-5)\)
\(\therefore\) correct option is (a).
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