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Q. Assume that the Earth rotates in a circular orbit round the Sun in 365 days. If the mass of the sun gets doubled but the radius of the orbit remains unchanged, the length of the year would be approximately

WBJEEWBJEE 2016

Solution:

$T=2\pi\sqrt{\frac{R^{3}}{GM}}$
$T\propto\frac{1}{\sqrt{M}} \left(for\, given \,orbital \,radius\right)$
$\frac{T_{1}}{T_{2}} =\frac{\sqrt{2M}}{\sqrt{M}}$
$T_{2}=\frac{T_{1}}{\sqrt{2}}=\frac{365}{\sqrt{2}}\approx258 days$