Q- A raindrop is observed at time t=0 when it has mass m and downward velocity u. As it falls under gravity its mass increased by condensation at a rate k and a resisting force acts on it proportional to its speed and equal to kv when the speed is v. Show that
d(M² v)/dt =M²g, where M = m +kt.

(On request from Abhishek)

Solution: