1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Fluids (Find the Force Exerted)

  1. Nov 20, 2004 #1
    Need some assistance on what I am missing
    "A tank contains mercury, whose density is 13 600 kg/m3. Find the force exerted by the mercury on a circular plug at the bottom of the tank. The plug has a diameter of 2.54 cm, and is located 37.6 cm below the surface of the mercury". The answer is 25.4 N

    1) I have tried p=F/A ending with A*p=F or 2.54*136000=F (Wrong)
    2) and F=phgA and again the number to big
    3) Finally I tried P2=P1+pgh with P1=(1.01x10^-5) again too large of an answer

    Am I missing something from try #1?

    Thanks for any assistance
     
  2. jcsd
  3. Nov 21, 2004 #2

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Your first attempt is wrong because you used the density in the formula for pressure! :eek: (it's ok...an honest mistake).

    pressure (letter 'p'): force per unit area

    [tex] p = \frac{F}{A} [/tex]

    density (letter 'rho'): mass per unit volume

    [tex] \rho = \frac{m}{V} [/tex]

    I'm curious...where did you get the answer 25.4 N? Is it given in the book? Because it doesn't specify what the pressure is above the surface of the tank. Assuming it was atmospheric pressure, and using the third formula you tried, I got the wrong answer. But assuming it (p1) was zero, I got the following:

    [tex] p_2 = 0 + \rho g h [/tex]

    [tex] F = pA = (\rho g h)(\pi r^2) [/tex] = (13 600 kg/m3 )(9.81 N/kg)(0.376m)([itex]\pi[/itex])((0.0254m)/2)2 = 25.4 N

    That's the right answer, but the method seems a little off...does the problem give more info about the pressure above the tank?
     
  4. Nov 21, 2004 #3

    Clausius2

    User Avatar
    Science Advisor
    Gold Member

    You solved it right, cepheid. Below the plug, there is atmospheric pressure too. So that, the force exerted by the atmosphere is cancelled at the two sides.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Fluids (Find the Force Exerted)
Loading...