Accelaration due to gravity of the earth

Let us learn what is accelaration due to gravity of the earth,
The earth can be imagined to be a sphere made of a large number of concentric spherical shells
with the smallest one at the centre and the largest one at its surface. A point outside the
earth is obviously outside all the shells. Thus, all the shells exert a gravitational force at the
point outside just as if their masses are concentrated at their common centre according
to the result stated in the last section. The total mass of all the shells combined is just the mass
of the earth. Hence, at a point outside the earth, the gravitational force is just as if its entire mass
of the earth is concentrated at its center. For a point inside the earth, the situation is different. This is illustrated in Fig.The mass m is in a mine located at a depth d below the surface of the Earth of mass ME and radius RE. We treat the Earth to be spherically symmetric.
Again consider the earth to be made up of concentric shells as before and a point mass m situated at a distance r from the centre. The point P lies outside the sphere of radius r. For
the shells of radius greater than r, the point P lies inside. Hence according to result stated in
the last section, they exert no gravitational force on mass m kept at P. The shells with radius ≤ r
make up a sphere of radius r for which the point
P lies on the surface.
Hope the above explanation helped you,Now let us learn the laws of gravity.