Where does the earth's gravitational field have the greatest strength?

• The equation for universal gravitation is given as: 
  $\mathrm{F} = \frac{{\mathrm{GM}}_1{\mathrm{M}}_2}{{\mathrm{R}}^2}$
• Here, G is the gravitational constant;  M₁ and  M₂ are the masses of the objects; R is a separation between the objects.
• The gravitational field between two bodies is inversely proportional to the square of the distance between their centres of gravity i.e. the higher the distance between the bodies the lower the gravitational field. 
• The Earth is not a perfect sphere and its radius varies at the poles and the equator, the radius being more at the equator than at the poles. 
• This means that a body at the equator is farther away from the earth’s centre than a body at the poles. Therefore, the gravitational field at the equator will be lesser than that at the poles.

Hence, the strength of the earth's gravitational field is maximum at its poles.