1. The problem statement, all variables and given/known data A spherical drop of water evaporates at a rate proportional to its exposed surface area as it moves.Set up its differential equation of motion.Find the time needed for complete evaporation.What is the velocity at that time? 2. Relevant equations 3. The attempt at a solution The volume of the sphere is : V=(4/3)πr^3, and the area is A=4πr^2 dV/dt = -k A,the minus sign is to indicate that the volume is decreasing dV/dt = (4/3)π(3)r^2dr/dt = 4πr^2dr/dt = -k(4πr^2), and simplifing this: dr/dt = - k Am I correct upto this?