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Homework Help: Force needed to pull out a plug out of a drain under water?

  1. Jun 15, 2016 #1
    1. The problem statement, all variables and given/known data
    The plug for the drain of a water tower is located 3.0 m below the surface. The plug is circular and has a diameter of 5.0 cm. If the density of water is 1000 kg/m3 , how much force is required to remove the plug (you may ignore the weight of the plug itself)
    2. Relevant equations
    A= pi * r ^2
    F=P *A

    3. The attempt at a solution
    A = pi * r^2 = pi * (0.025)^2 =1.96 * 10 ^-3 m^2

    F= pGH * A
    = (1000 kg/m3) (9.8) (3.0 m) * 1.96 * 10 ^-3 m^2
    = 57.624 N

    is this correct? there is no pressure from the atmosphere right?
  2. jcsd
  3. Jun 15, 2016 #2
    The atmospheric pressure is in addition to the 58N to give the total pressure from above. The pressure from below is just atmospheric. So the atmospheric pressure cancels.
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