1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Physics final prep, question about force under water at depth h

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

    The question states: The fresh water behind a reservoir dam has depth D=15 meters. A horizontal pipe 4.0cm in diameter passes through the dam at depth d=6.0m. A plug secures the pipe opening. (a) Find the magnitude of the frictional force between plug and pipe wall.

    Initially when I did this problem I found the pressure at this point with p = p(atm) + ρgh, where h = 6.0 meters. I then used the Force equation F=pA and used the area of the circular pipe with radius .02 meters. So my friction force, which is in equilibrium, should be equal to the force pushing from the water, F= (p(atm) + ρgh) * πr^2.

    However, when I look at the solutions manual, they have F = ρgh * πr^2. I'm not sure I understand why this is the case -- it seems to me that they are using the gauge pressure here to determine the force of the water. Why would we use the pressure as ρgh and not p(atm) + ρgh?
  2. jcsd
  3. May 9, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That's because there is atmopherisc pressure on the 'dry' side of the plug...'canceling' out the P_atm force part of the absolute water pressure....such that the net pressure on the plug is just the gauge pressure
  4. May 9, 2012 #3
    Ahh, pressing on the other side of the plug. Thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook