1. The problem statement, all variables and given/known data In a tropical rainstorm, 40mm of rain fell in one hour. Assuming that the raindrops struck an adequately drained roof normally with an average speed of 10m/s, find the pressure exerted on the roof by the rain. 2. Relevant equations (delta)p = F(delta)t 3. The attempt at a solution (delta)p = 10 x mass of rainfall striking the roof = F(roof on raindrop) x (delta)t F(roof on raindrop) = F(raindrop on roof) Pressure = F(raindrop on roof) / Area of the roof I have difficulty deriving the expression for mass of raindrop striking the roof. I also do not understand how is "44mm of rain" relevant to the question.