Question

f =mg describe this equation & it's derivation

Answer 1

Force is directly proportional to mass
That is F α m
   for example you pushed two blocks of different masses So you apply more    force on object having more mass than lighter one

And force is also directly proportional to acceleration
That is F α a
   for example if you kick a ball faster than you apply more force than applied     to kick slower

so F α ma
    F = ma
But in case of gravity due to acceleration a is g
Therefore F = mg

is this helpful

Answer 2

in case of gravity a=g
F=ma
so F=mg