# Air pressure with bernoulli's equation

Nevermind, I think I figured out why I needed to set P2 = 0 in the first problem. Actually, what I needed to do was set P2 = 1 atm, and when I calculated force, I needed to use F = (P2 - P1) * A to get the right answer.

So just ignore this...but here's the post anyway:

For anyone who has the book, giancoli 5th ed, pg 306 number 39 and 41.

Question 1: If wind blows at 30 m/s over your house, what is the net force on the flat roof if its area is 240 m^2.

Question 2: Estimate the air pressure at the center of a hurricane with wind speed of 300 km/h at the center.

For question 1 and 2, using bernoulli's equation, and setting y1=y2=0, we have:

P1 + 1/2*d*v1^2 = P2 + 1/2*d*v2^2 where d = density of air = 1.29 kg/m^3

For question 1, if we set P2 = 0 and v2 = 0, and v1 = 30 m/s, I get the right answer. I solve for P1 and then for the force, I just say F = P1 * A where A = 240m^2.

But for question 2, if I do the same thing, with P2 = 0 and v2 = 0 and v1 = 300 km/h = 83 m/s, I do not the right answer. But, if I set P2 = 1 atm, I get the right answer.

Can someone explain how to solve these two questions?

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