Give reasons:
$(i)$ What is meant by acceleration due to gravity?
$(ii)$ The value of $g$ is not constant on the surface of the Earth.
$(iii)$ Objects of different masses take the same time to fall from a fixed height.
$(iv)$ $G$ is known as the universal gravitational constant. Write its $SI$ unit.

(N/A) $(i)$ Acceleration due to gravity is the acceleration produced in a body due to the gravitational pull of the Earth.
$(ii)$ The Earth is not a perfect sphere; its radius is greater at the equator and smaller at the poles. Since $g = \frac{GM}{R^2}$,the value of $g$ varies with the radius $R$,making it higher at the poles and lower at the equator.
$(iii)$ The acceleration of a freely falling body is $g = \frac{GM}{R^2}$. This expression does not depend on the mass of the falling object $(m)$. Therefore,all objects fall with the same acceleration regardless of their mass,taking the same time to reach the ground from a fixed height.
$(iv)$ $G$ is called the universal gravitational constant because its value remains the same throughout the universe. Its $SI$ unit is $N \ m^2 \ kg^{-2}$.