NEET 2025 — Physics solution

NEET2025Physics

The Sun rotates around its centre once in $27$ days. What will be the period of revolution if the Sun were to expand to twice its present radius without any external influence? Assume the Sun to be a sphere of uniform density.

A
100 days
B
105 days
C
115 days
D
108 days

Solution

With no external torque, angular momentum is conserved: $I_{1} ω_{1} = I_{2} ω_{2}$ .

For a uniform sphere, $I = \frac{2}{5} M R^{2} \propto R^{2}$ . If $R_{2} = 2 R_{1}$ , then $I_{2} = 4 I_{1}$ , so $ω_{2} = ω_{1} /4$ .

Since $T \propto 1/ ω$ , $T_{2} = 4 T_{1} = 4 \times 27 = 108$ days.

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