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Homework Help: Statics problem about an anchored plank in water

  1. May 21, 2012 #1
    Hello, I'm new and quite unfamiliar with all the conventions, but I'll use underline for vectors since I couldn't find the proper thing (is the convention to use bold here?)

    No more.

    Relevant equations:
    Fnet = ma = 0
    τnet = Iα = 0
    τ = r x F = rFsinθ
    Fbuoyancy = ρwaterVg

    Attempt at solving:
    What I realised is that since the plank isn't in translational nor rotational motion, the net force and torque are both zero. T = tension, G = gravitational f., B = buoyancy.

    T + G + B = 0

    in scalar:

    T + G - B = 0

    Also, I know how to equate the torques (only gravity and buoyancy cause torque), but I can't find out the distances nor angles. I can also express the gravitational force in terms of density of the plank and it's volume...? Things just don't cancel out neatly there.

    I can provide a diagram if really necessary, but I'd prefer not to now, I have very limited time. Thanks in advance. Also, is the template there to be followed rigorously or is any kind of organised posting viable?
    Last edited: May 21, 2012
  2. jcsd
  3. May 21, 2012 #2

    Compute center of pressure as a function of angle and enforce equilibrium using moment summation.
  4. May 21, 2012 #3
    I would really like to have a solution given, or at least all the steps necessary, because I've been fighting with this for a good while and I don't feel like anymore, as it's not the only thing I need to know for Wednesday.

    If nobody gives me the answer before tomorrow, I might try it then... but I would really appreciate a solution, as I've tried to do this.

    E: Just noticed that complete solutions violate the rules... well is it possible to at least give me the steps, and I could do them myself?
    Last edited: May 21, 2012
  5. May 21, 2012 #4
    Find the upward force due to the portion under water as a function of the angle and the distance from the surface. That force is directed vertically and is applied at the center of gravity of the portion under water. This creates a moment. Determine the moment in the other direction due to its natural weight. The plank is in equilibrium so what do you know about the sum of the moments?
    Last edited: May 21, 2012
  6. May 21, 2012 #5
    L = length of plank

    the angle θ between the forces and the moment arm is the same for both as the forces are only antiparallel.

    B = ρwaterVsubmergedg
    G = ρplankVplankg

    V = ?

    τB = rBsinθ = (L/2 - x) * ρwaterVsubmergedg sinθ
    τG = rGsinθ = (L/2) * ρplankVplankg sinθ

    (L/2 - x) * ρwVsubg sinθ = (L/2) * ρpVpg sinθ

    → (L/2 - x) * ρwVsub = (L/2) * ρpVp

    Sum of the moments is zero, that I knew. I can identify all the forces and where they act, I just cannot make them into reasonable equations. Like volume, how do I get rid of it?
    Last edited: May 21, 2012
  7. May 21, 2012 #6
    The volume consists to a cross section multiplied by length. The cross section is arbitrary but appears in each term so it cancels.
  8. May 21, 2012 #7
    Oh, foolish me... I was stuck at the T + G - B = 0 with no hope of getting rid of V or A... had I only started with the moments.

    Thanks, I think it makes sense now, I'll try it and see if I get the answer.

    E: Yes, I got the answer, thank you. I think the angle is a matter of geometry.
    Last edited: May 21, 2012
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