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Homework Help: Pressure problem.

  1. Dec 9, 2012 #1
    1. The problem statement, all variables and given/known data

    A creamy salad dressing is made up of heavy cream, corn oil, and vinegar (as well as a pinch of dry mustard, salt and pepper). You put the cream, oil, and vinegar in a glass jar. The density of the cream is 994 kg/m3, the oil 880 kg/m3, and the vinegar 1000 kg/m3. The salad dressing liquids separate into three layers. (a) What is the order of the layers from top to bottom? (b) If the cream layer is 2.20×10−2 m tall, the oil layer 2.80×10−2 m tall, and the vinegar layer 1.60×10-2 m tall, what is the gauge pressure at the bottom of the jar?

    2. Relevant equations

    Patm= 1.013 x 10^5 Pa

    P=F/Area or P= Density x gravity x height

    3. The attempt at a solution

    I got a) Oil, cream, vinegar because the one with the least density would float on top etc.

    B confuses me because I don't understand how to calculate Gauge Pressure
  2. jcsd
  3. Dec 9, 2012 #2
    Gauge pressure is the absolute pressure minus atmospheric pressure.
  4. Dec 9, 2012 #3
    How would you find absolute with all of those, would you calculate the pressure of each layer, add them together and subtract atmospheric? Or is Atm. not in it already?
  5. Dec 9, 2012 #4
    The pressure above the top layer is 1 atm. Your equation for pressure is not correct. It should read

    change in pressure over height of layer = density x gravity x height
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