# Homework Help: Question on impulse

1. Apr 9, 2010

### gaobo9109

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.

2. Apr 9, 2010

### jeppetrost

What you need the "40 mm of rain" for is to figure out how much rain falls in a given timespan.
From the 40mm (meaning 40 liters per square meter per hour) of rain, obtain the mass of the falling water (per second per square meter). Now, you know how much water falls (the mass), and you know how fast it is falling, so you can calculate the pressure from the equation
$$P = F/A = \frac{dp}{dt}/A = \frac{dm/dt}{A} v$$
as you can see, you will have all the necessairy quantities.