Assuming the earth to be a sphere of uniform mass density, how much would a body weigh half way down to the center of the earth if it weighed 250 N on the surface?


The weight of a body is the force acting one it due to earth’s gravity and is the product of mass and acceleration due to gravity is given as

W = mg


Where W is the weight of a body of mass m on the surface of the earth, g is acceleration due to gravity on the surface of the earth


Now as the body goes inside surface of earth towards the centre of earth acceleration due to gravity decreases, so the weight of the body also decreases i.e.


Wd = mgd


Where Wd is the weight of a body of mass m kept at a depth d inside the surface of the earth, Let gd be the acceleration due to gravity at a at a depth d inside the surface of the earth


Now acceleration due to gravity at a depth d inside the surface of the earth is given by the relation



Where gd is the acceleration due to gravity at a depth d inside the surface of the earth, g is acceleration due to gravity on earth’s surface and R is Radius of the Earth


The situation has been shown in figure



Now Let gd be the acceleration due to gravity half way down to the centre of the earth and g be the acceleration due to gravity on the surface of the earth.


If R is the radius of the Earth and mass in half way down the earth then we have the depth of body from the surface of earth


d = R/2


Then using



We get



gd = g/2


i.e. acceleration due to gravity half way down the surface of the earth is half of that on the surface of the earth


so the weight of the body at this depth will be


Wd = mgd


Wd = m(g/2) = 1/2 mg


i.e. Wd = 1/2 W


now we are given the weight of the body on the surface of the earth as


W = 250 N


i.e. we have


Wd = 250/2 N


Wd = 125 N


So, the weight of the body at a depth equal to half the radius of the earth is 125 N.

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