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!

Buoyant Force of a ship and water

  1. Apr 19, 2013 #1
    1. The problem statement, all variables and given/known data
    Consider a ship that is floating in fresh water. The bottom of the ship is a depth of (df) below the surface. If the same ship is floating in sea water, the bottom of the ship (ds) below the surface. Given that the density of sea water is greater than the density of fresh water, which one of the following statements is correct?

    The correct answer is ds<df


    2. Relevant equations

    Buyont Force = B=pVg

    3. The attempt at a solution

    B=(pf)(Vf)g=(ps)(Vs)(g)

    Vf>Vs ---> How was this simplified to give the correct answer. I am confused.
    df>ds
     
  2. jcsd
  3. Apr 19, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    How does volume of water displaced relate to depth below the surface?
     
  4. Apr 19, 2013 #3
    Volume relates to depth below the surface in the equation (Pressure under water: P=ρ•g•h ; where ρ is the density of water m/V and h is the depth of water)
     
  5. Apr 19, 2013 #4

    Doc Al

    User Avatar

    Staff: Mentor

    That equation describes the pressure beneath the surface of a fluid. What I asked for was much simpler. If a ship displaces a greater volume (Vf > Vs, say), what can you say about the depth of its bottom beneath the surface? (It's an easy question--don't over think it.)
     
  6. Apr 19, 2013 #5
    That they are proportional. I am still confused as to how from the Buoyant force equation we were able to come up with the result of Vf being greater then Vs.
     
  7. Apr 19, 2013 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Ah, now I understand your question.

    I think you understood this:
    (pf)(Vf)g=(ps)(Vs)(g)

    Now just rearrange as ratios, so that (pf)/(ps) = ???

    How? Divide both sides by g, then by Vf, then by ps.
     
  8. Apr 19, 2013 #7
    So if the density of Ps is greater then Pf, and not knowing by how much; how exactly would I set up the ratio? Also the ratio of Vs and Vf --> Would it be something like this? [(Pf)/(Ps)] [(Vf)/(Vs)]=
     
  9. Apr 19, 2013 #8

    Doc Al

    User Avatar

    Staff: Mentor

    Start with the equation I gave in the last post. (Which was from your first post.)

    Then do each of the three steps I outline.

    1) Divide both sides by g
    ... and so on.
     
  10. Apr 19, 2013 #9
    I am more of a visual learner as to where what goes. Still confused, but i'll ask someone from my department at school. Thanks!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted