Gravitation

Given, 
Mass of the planet = ${\mathrm{M}}_{\mathrm{p}}$
The diameter of the planet = ${\mathrm{D}}_{\mathrm{p}}$
Mass of the freely falling particle = m 
Force, 
$\mathrm{F}=\frac{{\mathrm{GM}}_{\mathrm{p}}\mathrm{m}}{{{\mathrm{R}}_{\mathrm{p}}}^2}$ 
Here, $\mathrm{F}=\frac{{\mathrm{GM}}_{\mathrm{p}}\mathrm{m}}{{({\mathrm{D}}_{\mathrm{p}}/2)}^2}$ 
$\text{F=}\frac{4{\mathrm{GM}}_{\mathrm{p}}\mathrm{m}}{{\mathrm{D}}_{\mathrm{p}}^2}$
Acceleration due to gravity, 
$\mathrm{a}=\frac{\mathrm{F}}{\mathrm{m}}=\frac{4{\mathrm{GM}}_{\mathrm{p}}}{{\mathrm{D}}_{\mathrm{p}}^2}$