MCQ
Three identical bodies of equal mass $M$ each are moving along a circle of radius $R$ under the action of their mutual gravitational attraction. The speed of each body is
  • A
    $\sqrt {\frac{GM}{R}}$
  • B
    $\sqrt {\frac{GM}{3R}}$
  • $\sqrt {\frac{GM}{\sqrt 3 R}}$
  • D
    $\sqrt {\frac{GM}{\sqrt 2 R}}$

Answer

Correct option: C.
$\sqrt {\frac{GM}{\sqrt 3 R}}$
c
Consider the circle with centre at $\mathrm{O},$ and having radius $\mathrm{R}$. In it consider three bodies of masses $M$ each moving with velocity $v$ under the action of their gravitational attraction.

$\mathrm{R}=\frac{2}{3} \mathrm{AD}=\frac{2}{3} \times \mathrm{L} \sin 60^{\circ}=\frac{2 \mathrm{L}}{3} \times \frac{\sqrt{3}}{2}$

$=\frac{\mathrm{L}}{\sqrt{3}} \quad$ or $\quad \mathrm{L}=\sqrt{3} \mathrm{R}$

Centripetal force on any one mass $\mathrm{M}$ is

$=\frac{2 \mathrm{GM}^{2}}{\mathrm{L}^{2}} \cos \left(\frac{60^{\circ}}{2}\right)$

$\therefore \frac{\mathrm{Mv}^{2}}{\mathrm{R}}=\frac{2 \mathrm{GM}^{2}}{\mathrm{L}^{2}} \times \frac{\sqrt{3}}{2}$

or $\frac{\mathrm{GM}^{2}}{(\sqrt{3} \mathrm{R})^{2}} \sqrt{3}=\frac{\mathrm{Mv}^{2}}{\mathrm{R}}$

$\therefore v=\sqrt{\frac{\mathrm{GM}}{\sqrt{3} \mathrm{R}}}$

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