• # question_answer 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: [JEE Main 9-4-2019 Morning] A) $\frac{2GM}{9{{a}^{2}}}$                      B) $\frac{GM}{3{{a}^{2}}}$C) $\frac{GM}{9{{a}^{2}}}$                        D) $\frac{2GM}{3{{a}^{2}}}$

We use gauss's Law for gravitation $g.4\pi {{r}^{2}}=$(Mass enclosed)$4\pi G$ $g=\frac{3M4\pi G}{4\pi {{(3a)}^{2}}}$$=\frac{MG}{3{{a}^{2}}}$ Option [b]