Question

due to acceleration due to gravity, every body falling on earth falls with constantly increasing speed.But why does a rain drop falls with a constant velocity?

Answer 1

Firstly let me tell you that the answer of this question is hidden in Fluid mechanics than in gravitation!
Because while doing Gravitational questions in Kinematics we basically neglect the Air Drag but for rain drops it is a very powerful force.
There is a term called Viscosity which is a kind of force that is directly proportional to the velocity of body and acts in different direction to that of object's velocity.

F = 6πηrv ....... (1) here η is the coefficient of viscosity
r is the radius of the spherical rain drop
v is its velocity
now when the rain drop starts falling under the action of gravity initially its velocity increases,
as its velocity increases the Viscous force (F) also increases but in opposite direction to that of mg force
now as the velocity goes on increasing a point comes when mg = F( viscous force)
now the net force on the rain drop is zero
now it is having no acceleration and it falls with a constant velocity towards earth which we call as TERMINAL VELOCITY !!