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Homework Help: Fluids Physics Question

  1. Apr 17, 2010 #1
    1. The problem statement, all variables and given/known data
    A physics book has a height of 26.0 cm, a width of 22.4 cm, and a thickness of 3.10 cm.

    (a) What is the density of the physics book if it weighs 18 N?
    ??? kg/m3
    (b) Find the pressure that the physics book exerts on a desktop when the book lies face up.
    ??? Pa
    (c) Find the pressure that the physics book exerts on the surface of a desktop when the book is balanced on its spine.
    ??? Pa

    2. Relevant equations

    Divide the weight by gravity to get the mass.
    18N/9.81 = 1.83 kg.
    P = F/A

    3. The attempt at a solution

    Convert cm to meters:
    (26.0cm)/100 = 0.26
    22.4/100 = 0.224
    3.10/100 = 0.031
    Now divide
    1.83/(0.26*0.224*0.031) = 1.83/0.00180544 = 1014 kg/m^3
    P = F/A
    area of the book = (0.224*0.26) = 0.5824 m^2
    18/0.5824 = 30.9 Pa

    When balanced on its spine, its width and thickness make up the area
    (0.224*0.0310) = 0.006944
    18/0.006944 = 2592 Pa
  2. jcsd
  3. Apr 17, 2010 #2
    You got the first two right, but I would say that you should use height in the last problem instead of width.
  4. Apr 18, 2010 #3
    Like i said, I believe my answer is wrong. If someone would double check it.
  5. Apr 18, 2010 #4
    That's what I'm saying. Your answer is wrong because you used the wrong number in your calculations. All you need to do is use the right one.
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