JEE MAIN - Physics (2022 - 26th July Morning Shift - No. 20)
A disc of mass $$1 \mathrm{~kg}$$ and radius $$\mathrm{R}$$ is free to rotate about a horizontal axis passing through its centre and perpendicular to the plane of disc. A body of same mass as that of disc is fixed at the highest point of the disc. Now the system is released, when the body comes to the lowest position, its angular speed will be $$4 \sqrt{\frac{x}{3 R}} \,\operatorname{rad}{s}^{-1}$$ where $$x=$$ ____________. $$\left(g=10 \mathrm{~ms}^{-2}\right)$$
Answer
5
Explanation
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Loss in P.E. = Gain in K.E.
$$2mgR = {1 \over 2}\left[ {{1 \over 2}m{R^2} + m{R^2}} \right]{w^2}$$
$$2mgR = {1 \over 2} \times {3 \over 2}m{R^2}\,{w^2}$$
$${w^2} = {{8g} \over {3R}}$$
$$w = \sqrt {{{8g} \over {3R}}} = 4\sqrt {{g \over {2 \times 3R}}} $$
$$ \Rightarrow x = {g \over 2} = 5$$
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