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Calculating pressure exerted by a cylinder

  1. Feb 4, 2016 #1
    1. The problem statement, all variables and given/known data
    A 75 kg solid cylinder, 2.1 m long and has an end radius of 5.0 cm, stands on one end. How much pressure does it exert?

    2. Relevant equations
    P = f/a
    P= pwh (this is the equation my teacher gave us on a power point but I might just be using it wrong)
    v = pi r2h



    3. The attempt at a solution

    v = .052 pi 2.1
    v = .016 m3

    density = 75 / .016
    density = 4687.5


    weight = 75 x 9.8
    weight - 735

    P = 735 x 4687.5 x 9.8
    P = 33764062.5

    I feel like that number is way to big and I am doing more work than is needed. Thanks in advance!



     
  2. jcsd
  3. Feb 4, 2016 #2

    andrewkirk

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    Yes you do appear to be doing more work than needed. From your first equation P = F/A you can see that you only need the downward force and the area. The force is obtained directly from the given 75kg mass and the acceleration due to gravity, via F=ma. The area A is simply the area of the circular end of the cylinder.

    The only reason to use the volume of the cylinder would be if you were to make an adjustment for the buoyancy of the cylinder in air. But since we do not know if the cylinder is in air (and at what air pressure) or in a vacuum, we cannot make that adjustment.

    It will still be a big number, as it is expressed in Newtons per square metre and Newtons are small while square metres are big. But it won't be quite as big as the number you got.
     
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