A body weighs $$72\ N$$ on the surface of the earth. What is the gravitational force on it, at a height equal to half the radius of the earth ?
Correct option is A. $$32\ N $$
$$W_{s} = mg_{s} = 72\ N $$
$$W_{h} = mg_{h} = \dfrac{mg_{s}}{\left (1 + \dfrac{h}{R} \right )^{2}} = \dfrac{72\ N }{\left (1 + \dfrac{R/2}{R} \right )^{2}} = \dfrac{72}{9/4} $$
$$W_{h} = 32\ N $$