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7.Gravitation
easy
A body weighs $72 N$ on surface of the earth. When it is taken to a height of $h=2 R$, where $R$ is radius of earth, it would weigh ........ $N$
A
$36$
B
$18$
C
$9$
D
$8$
Solution
(d)
Weight on earth $=m g=m \times \frac{G M}{R^2}=72 \,N$
Weight at height, $h=2 R$ will be $m g^{\prime}=m\left(\frac{G M}{r^2}\right)=m \times \frac{G M}{(R+2 R)^2}=\frac{G M m}{9 R^2}=\frac{72}{9}=8 \,N$
Standard 11
Physics
