According to Newton's Law of gravitational, every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them. [Fig. (1)]

$\mathrm{F}=\left(\frac{{\mathrm{Gm}}_1{\mathrm{m}}_2}{{\mathrm{r}}^2}\right)$             (where G is the universal gravitational constant)

Now, If m₁ is earth's mass M and m₂ is the mass of block m kept on the earth's surface. The distance between the center of the earth to the block is equal to the radius of the earth R.
∴ Force on the block, $\mathrm{F}=\frac{\mathrm{GMm}}{{\mathrm{R}}^2}$
Where R is the radius of the earth. [Fig. (2)]