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JEE MAIN 2021
09-01-19 S2
Question
Expression for time in terms of G(universal gravitational constant), h (Planck constant) and c (speed of light) is proportional to:
Select the correct option:
A
$ \sqrt{\frac{G h}{c^5}} $
B
$\sqrt{\frac{c^3}{G h}}$
C
$\sqrt{\frac{G h}{c^3}}$
D
$\sqrt{\frac{c^3}{G}}$
✓ Correct! Well done.
✗ Incorrect. Try again or view the solution.
Solution
$\begin{aligned} & {[\mathrm{T}]=[\mathrm{G}]^{\mathrm{a}} \cdot[\mathrm{h}]^{\mathrm{b}} \cdot[\mathrm{c}]^{\mathrm{c}}} \\ & =\left[\mathrm{M}^{-1} \mathrm{~L}^3 \mathrm{~T}^{-2}\right]^{\mathrm{a}}\left[\mathrm{ML}^2 \mathrm{~T}^{-1}\right]^{\mathrm{b}}\left[\mathrm{LT}^{-1}\right]^{\mathrm{c}} \\ & -\mathrm{a}+\mathrm{b}=0 \\ & 3 \mathrm{a}+2 \mathrm{~b}+\mathrm{c}=0 \Rightarrow 5 \mathrm{a}+\mathrm{c}=0 \\ & -2 \mathrm{a}-\mathrm{b}-\mathrm{c}=1 \Rightarrow 3 \mathrm{a}+\mathrm{c}=-1 \\ & \Rightarrow \mathrm{a}=\frac{1}{2}, \quad \mathrm{~b}=\frac{1}{2}, \quad \mathrm{c}=\frac{-5}{2} \\ & \Rightarrow[\mathrm{~T}]=\sqrt{\frac{\mathrm{Gh}}{\mathrm{c}^5}}\end{aligned}$
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