on an object falling we have three forces acting mainly
force of gravity : acts in downward direction
force of buoyancy : acts in upward direction
drag force due to air : acts in upward direction
the force of gravity makes the object fall in downward direction and force of buoyancy acts in opposite upward direction to oppose the motion. initially since force of gravity was greater than the force of buoyancy , the net force is acting in down direction and hence the object gains speed in downward direction.
on the object falling down , drag force by air acts in upward direction to oppose is downward motion. the drag force is directly propotional to the square of the speed. hence as the object gains speed in down direction , the drag force increase in upward direction.
at some velocity called terminal velocity , the sum of drag force and force of buoyancy becomes equal to the force of gravity and then there is no net force on the object . so the object then moves at constant terminal velocity.
at terminal velocity ,
force of gravity = force of air resistance + force of buoyancy