Question

Value of g is constant,but when we throw a stone and a feather from a certain height,stone reach the ground fast?

Answer 1

In vacuum both stone and feather fall on the ground from a height with the same acceleration and speed.

Let the mass of a stone by m.  Let the density be d_s.  So volume is m/d_s.  Let d_air be the density of air.

The force acting on the stone is mg and the buoyancy force acting on the stone is m/d_s * d_air * g.
  Net force downwards =  m g (1 - d_air/d)
  Acceleration = a_s = g (1 - d_air/d_s)

Similarly for the feather, acceleration downwards = a_f = g ( 1 - d_air/d_f)

As d_s > d_f,  a_s > a_f.

The reason is buoyancy force and density.