Question

Why does the velocity of rain drop attain constant value because of?

Answer 1

The forces acting on a Rain drop while falling down are : (1) Its weight, W = mg and  

(2) Air resistance, R = k v², where v is the speed of the drop and k is a constant.  

Net force acting on the rain drop = F = W - R = ma, where a = acceleration. Now,  

W is constant whereas R is variable, inreasing with the speed of the drop. This causes the net force  

and the net acceleration to decrease. As R gradually increases from zero value, a stage comes  

when R = W.  

At that stage the net force on the rain drop is zero and consequently, the acceleration is zero.  

By the Newton's 1st law of motion the rain drop continues to move with a constant velocity, known  

as the Terminal velocity from that instant till it hits the ground or other objects.