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A body weighs $144 \,N$ at the surface of earth. When it is taken to a height of $h=3 \,R$, where $R$ is radius of earth, it would weigh .......... $N$
$48$
$36$
$16$
$9$
Solution
(d)
Given that,
Weight of body $W =144\,N$
Height $h=3 R$
Radius of earth $= R$
We know that,
Acceleration due to gravity at the surface of the earth $= g$
Acceleration due to gravity at height $g ^{\prime}=3 R$
Now,
$\frac{ g }{ g ^{\prime}}=\left(\frac{ R }{ R + h }\right)^2$
$\frac{ g }{ g ^{\prime}}=\left(\frac{ R }{ R +3 R }\right)^2$
$\frac{ g }{ g ^{\prime}}=\frac{1}{16}$
Now, weight at the height
$\frac{ w ^{\prime}}{ w }=\frac{ mg }{ mg }$
$\frac{w^{\prime}}{w}=\frac{1}{16}$
$w^{\prime}=\frac{w}{16}$
$w^{\prime}=\frac{144}{16}$
$w^{\prime}=9\,N$
Hence, the weight at the height is $9\,N$