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!

Pressure on a Dam (go figure)

  1. Nov 4, 2007 #1
    Water stands at a depth D=35 m behind the vertical upstream face of a dam of width W=314 m.

    Find the net horizontal force on the dam from the gauge pressure of the water:

    I am quite lost here. I thought it would just be F=Pressure*Area
    =(rho*g*D)(D*W). But that is horribly wrong. What am I missing?

  2. jcsd
  3. Nov 4, 2007 #2


    User Avatar

    Staff: Mentor

    Well =(rho*g*D)(D*W) assumes constant pressure along D, which is not the case.

    The pressure increases with depth rho * g * x, and one can then use dA = W dx.

    The pressure at the surface of the water is pg = 14.7 psi, or 0.101325 MPa (i.e. 1 atm).

    The pressure as a function of depth is P(x) = pg + rho*g*x

    Try an integral F = Int (P(x) dA) = Int (P(x) W dx) with limits of 0 to D = 35 m.
  4. Nov 4, 2007 #3
    Hmm. I have never used integration in physics (yet:redface:)...I was just picking random problems to practice. this will be a nice first.

    So dA=W dx means a differential change in Area corresponds to one in depth. That is, W is constant and here you used "x" for depth.

    Now it says use the "gauge pressure" ...so I don't think I need the leading term of 1atm since gauge pressure is just the change due to depth.

    So F=Int[P(x)*W dx]
    =(rho*g*W)Int[ x dx] from x=0-->35

    WOW! That worked great! 1.88 *10^9 N. (correct)

    Thanks Astronuc!


    not to push my luck, but if you have a moment, maybe you could clear up my misunderstandings Here.:wink:

    Edit:nevermind, I got it using an integral! whoooaahhhh Integrals!
    Last edited: Nov 4, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Pressure on a Dam (go figure)
  1. Pressure pn a Dam (Replies: 1)

  2. Pressure on a Dam (Replies: 3)