Hurricane wind blowing across a flat roof

1. Jun 30, 2009

xKalax

1.A hurricane wind blows across a 7.8 m x 12 m flat roof at a speed of 180 km/hr.

What is the pressure difference?

How much force is exerted on the roof?

2. Relevant equations

P= p_atmos + pgd
P=F/A

3. The attempt at a solution
p = 101300Pa - (1.3)(9.8m/s^2)(93.6m^2)
p = 100107.5Pa

100107.5Pa = F/(93.6m^2)^2
F = 8.8 * 10^8 N

I know this is wrong, how do I go about solving this problem?

Last edited: Jun 30, 2009
2. Jun 30, 2009

jix

That's not the right equation. Rather, it's not the entire equation.

First of all, the numbers you used don't even turn out to have the right units. As in, d = 93.6 m^2 doesn't have the right units for d.

And think about it, the speed is given for a reason. Get the full equation, and plug in the values, and easily get the answer.

3. Jun 30, 2009

xKalax

ok, I got the change in pressure to be 1625 Pa, which is correct.

I plug that into F = pA and get F = 1625Pa * 93.6m^2 = 152100, this seems too big.

I got it right, thank you!

Last edited: Jun 30, 2009