Let us consider a body of mass M lying on the surface of earth of mass M and radius R. Let g be value of acceleration due to gravity on the free surface of earth.
Then,
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Suppose the body is taken to height 'h' above the surface of earth where the value of acceleration due to gravity is gh.
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Then
Where (R+h) is the distance between the centers of body and earth.
Dividing (ii) by (i), we get
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when h < < R, then

Since, h < < R, then h/r is very small as compared to I. Expanding the right hand side of the above equation by Binomial Theory and neglecting squares and higher powers of h/r, we get
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Thus, acceleration due to gravity decreases with increase in height / altitude.
