How will you arrive at the equation for acceleration due to gravity on earth?

From Newton's second law of motion we know that force is the product of mass and acceleration.
F = ma
If the force due to gravity on a particle of mass m has magnitude
F = mg
where g denotes the acceleration due to gravity, then by using the equation of force of gravitation
F =
we get
mg = G
g = G
where M is the mass of the earth, r is the distance between the particle and the center of the earth.
If the particle is on or near the surface of the earth, then r is taken equal to R, the radius of the earth. Thus for bodies on or near the surface of the earth,
mg = G
g = G.