The ratio of radii of gyration of a ring to a disc (both circular) of same radii and mass, about a…

The ratio of radii of gyration of a ring to a disc (both circular) of same radii and mass, about a tangential axis perpendicular to the plane is
  1. $\frac{2}{\sqrt{3}}$
  2. $\frac{\sqrt{2}}{1}$
  3. $\frac{\sqrt{3}}{\sqrt{2}}$
  4. $\frac{2}{\sqrt{5}}$

Solution

$I_{\text {Ring }}=M R^{2}+M h^{2}=M R^{2}+M R^{2}=2 M R^{2}$ $I_{\text {Disc }}=\frac{1}{2} M R^{2}+M h^{2}=\frac{1}{2} M R^{2}+M R^{2}=\frac{3}{2} M R^{2}$ $\therefore k_{\text {ring }}=\sqrt{2} R$ $\quad k_{\text {disc }}=\sqrt{\frac{3}{2}} R$ $\therefore \frac{k_{\text {ring }}}{k_{\text {disc }}}=\frac{\sqrt{2}}{\sqrt{3}}=\frac{2}{\sqrt{3}}$

Asked in: MHT CET 2020 (16 Oct Shift 2)

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