A) 72 N
B) 36 N
C) 18 N
D) zero
Correct Answer: C
Solution :
Key Idea: At a height h from earth?s surface, the value of acceleration due to gravity is \[g'=\frac{g}{{{\left( 1+\frac{h}{R} \right)}^{2}}}\] The acceleration due to gravity at height h from earth?s surface is \[g'=\frac{g}{{{\left( 1+\frac{h}{R} \right)}^{2}}}\] Weight of a body at height h \[w'=mg'\] \[=\frac{mg}{{{\left( 1+\frac{h}{R} \right)}^{2}}}=\frac{w}{{{\left( 1+\frac{h}{R} \right)}^{2}}}\] Now at height h equal to radius (R) of earth i.e., h = R, we have \[w'=\frac{w}{{{\left( 1+\frac{R}{R} \right)}^{2}}}\] \[orw'=\frac{w}{4}\] \[butw=72\,N(given)\] \[\therefore w'=\frac{72}{4}=18\,N\]
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