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Pressure difference when height changes

  1. Apr 7, 2013 #1
    1. The problem statement, all variables and given/known data

    Calculate the difference in pressure (in atmospheres) that the blood vessels in a giraffe's head have to accommodate as the head is lowered from a full upright position to ground level for a drink. The height of an average giraffe is about 6m.
    (Note I'm currently only looking for the answer is Pascals)


    2. Relevant equations

    I thought the equation was P=Po + pgh
    but the answer just says P=pgh

    Po= 101000Pa

    3. The attempt at a solution

    P=Po + pgh
    P=101000 + (1.05x10^3 kg/m^3) x 9.8 x 6m
    P=162.740Pa

    But the answer gives...

    ΔP=pgΔh
    [itex]\frac{ΔP}{Po}[/itex] = [itex]\frac{pgΔh}{Po}[/itex]
    = [itex]\frac{(1.05\times103)\times9.8\times6}{1.013\times105}[/itex]
    =0.609Pa

    I don't understand why they've rearranged the equation the way they have and exactly how they are using it. It just doesn't seem right.
    Can someone please explain?
     
  2. jcsd
  3. Apr 7, 2013 #2
    I thought the equation was P=Po + pgh

    I don't think so. this is the equation for absolute pressure.
    The problem asks for difference in pressure.
     
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