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Homework Help: Vacuum required to generate specific amount of force

  1. Jul 31, 2007 #1
    1. The problem statement, all variables and given/known data
    I have a circular vacuum cup. The cup has a diameter of 4.5 inches. I need to calculate the amount of pressure, in inches of mercury (Hg), that the vacuum created by the cup would have to be in order to generate 120 pounds of force on a flat object.

    2. Relevant equations
    None given.

    3. The attempt at a solution
    It's been years since I've had to do any math or physics, but this is what I think is correct.

    First, P = F/A, where P = pressure, F = force, and A=area
    P = 120 pounds / ((2.25)^2 * pi) = 7.55 psi

    Second, convert psi to Hg:
    1 psi = 2.036 Hg, so 7.55*2.036 = 15.36 Hg

    So, what I'm hoping for here is verification that I did this correctly or someone to set me straight on this.
  2. jcsd
  3. Jul 31, 2007 #2


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    Science Advisor
    Homework Helper

    Sounds right, you have to subtract the answer from atmospheric pressure ( about 15psi) to get the absolute pressure inside the cup.
    You have calculated the pressure difference.
  4. Aug 6, 2007 #3
    Thank you for you assistance on this.

    I also apologize for my delayed reply, I was on vacation.
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