Question

at what height above the surface of the earth the weight of a man will be 20kg if his weight on the surface of earth is 80kg

Answer 1

W = mg
g = GM/ (R+h) ^2
Here w1/w2=1/4=g1/g2=R^2/(R+h)^2
R/(R+h) =1/2
h =R = equal to earth radius

Answer 2

Given :

Weight of man in surface of earth , W = 80 kg .

To Find :

At what height above the surface of the earth the weight of a man will be 20 kg .

Solution :

We know , weight of object is inversely proportional to the square of distance from the centre .

Therefore ,

$\dfrac{20}{80}=\dfrac{R^2}{(R+h)^2}$

Here , R is radius of earth i.e R = 6400 km .

So ,

$\dfrac{1}{2}=\dfrac{R}{R+h}\\R+h=2R\\h=R\\h=6400 \ km$

So , at the height of 6400 km from the surface of earth weight of a man will be 20 kg.

Learn More :

Gravitation :

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