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Homework Help: Net force on roof from wind? Using Bernoulli's eq?

  1. Dec 15, 2009 #1
    1. The problem statement, all variables and given/known data
    If wind blows at 24 m/s over your house, what is the net force on the roof if its area is 300 m squared?


    2. Relevant equations
    Bernoulli's equation? P1+1/2pv1sqrd+pgy1=P2+1/2pv2sqrd+pgy2


    3. The attempt at a solution
    Really didn't know where to go with this. I remembered my teacher mentioning that Bernoulli's equation was relevant. Tried plugging in the numbers, but didn't have any success. Any help would be extremely appreciated, as I have a final on this tomorrow morning. Thanks
     
  2. jcsd
  3. Dec 15, 2009 #2

    ehild

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    Homework Helper

    In case of a horizontal roof, the terms containing the height y will cancel in Bernoulli's equation. The wind blows above the roof, the pressure is lower there than below the roof, in the building, where the air is in rest. If Po is the pressure of the air in rest, P+1/2 d v2=Po. The total force on the roof is (Po-P)A, (A is the area of the roof). You need the density of air (d).


    ehild
     
  4. Dec 15, 2009 #3
    Thank you so much for your help.
    from bernoulli's I get to .5(d)(v1sqrd)=P2-P1. Once I plug in my numbers: .5(1.293)(576)=372.384. Then multiplied by the area: 372.384(300)= 111715.2 N/msqrd. For some reason the study sheet has the answer as 108000 N/msqrd, but maybe this could just be rounding.
    Conceptually, I'm wondering so on the left side of the equation is the wind blowing 24m/s, while on the right side it is stopped? Thank you so much, you have been monumentally helpful.
     
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