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Homework Help: Estimating the force of he water on the dutch boys finger

  1. Sep 22, 2012 #1
    1. The problem statement, all variables and given/known data

    The little dutch boy saved HOlland by sticking his finger in the leaking dike. if the water level was 2.5m above his finger, estimate the force of the water on his finger

    2. Relevant equations

    3. The attempt at a solution

    I calculated the pressure on his finger by P = p0+pgh = 125,800 Pa.

    But i have no idea what to do now.
  2. jcsd
  3. Sep 22, 2012 #2
    Pressure is Force/Area. You must estimate the area of the boy's finger and use this to convert the pressure to force.
  4. Sep 22, 2012 #3
    how do i do that..
  5. Sep 22, 2012 #4
    Just look at the units:

    [itex]\displaystyle \frac{Force}{Area} × Area = Force[/itex]

    To estimate the area of the boy's finger, I would just measure my own finger. Measure the diameter of your finger and approximate it as a circle.
  6. Sep 22, 2012 #5


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    Nellen2222, Should you use absolute pressure or gauge pressure ρgh? (Edit: forget my question, I was thinking the question was asking what force the boy would need to exert to hold the water back.)
    Last edited: Sep 22, 2012
  7. Sep 22, 2012 #6


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    Estimate the diameter of his finger. Assume it's circular?
  8. Sep 22, 2012 #7
    Assume your finger is approximately a cylinder. Can you guess what the diameter of your finger is? Mine is about 1 cm. But if you can't even estimate that, wrap a string around your finger, and measure its length (after unwrapping it again). Then divide by Pi to get the diameter. Do you know how to get the area of a circle if you know its diameter? Your teacher is not looking for the exact answer, so you don't have to determine the diameter very accurately.
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