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JEE Main 2019 (9 Apr, Shift 1) · Gravitational Field

Gravitational Field question from JEE Main 2019 (9 Apr, Shift 1)

Q11JEE Main 2019 · 9 APR · SHIFT 1Medium

Gravitational Field

A solid sphere of mass 'M' and radius 'a' is surrounded by a uniform concentric spherical shell of thickness 2a and mass 2M. The gravitational field at distance '3a' from the centre will be:

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Answer: (a)

  1. The shell extends from aa to 3a3a, so at r=3ar = 3a the whole mass M+2M=3MM + 2M = 3M is enclosed.
  2. Enclosed mass acts as a point mass at the centre: E=G(3M)(3a)2E = \frac{G(3M)}{(3a)^2}
  3. E=3GM9a2=GM3a2E = \frac{3GM}{9a^2} = \frac{GM}{3a^2}
  4. So the answer is (a) GM3a2\frac{GM}{3a^2}.
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