Q. A man weighs 80 kg on earth surface. The height above ground where he will weigh 40kg, is (radius of earth is 6400 km)
A) 0.31 times $r$
B) 0.41 times $r$
C) 0.51 times $r$
D) 0.61 times $r$
Explanation
(b) Weight of body at height above the earth’s surface is
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ w^{'} = \frac{w}{\left( 1 + \frac{h}{r} \right)^{2}}$
$\Longrightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 40 = \frac{80}{\left( 1 + \frac{h}{r} \right)^{2}}$
$\Longrightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ h = 0.41r$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.