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Homework Help: Force required to produce wave in tank

  1. Nov 15, 2009 #1
    1. The problem statement, all variables and given/known data
    I am building a tank to produce waves.
    I have two options to power the flap that will be produced to create the waves:

    1. Pneumatic system
    2. Motor and Cam system

    Before I can order motors or pneumatic air cylinders I need to know what force will be required to move the flap the required distance to produce the waves.

    Sketches of both systems are attached
    Sketch of problem is attached

    2. Relevant equations

    Fxd = mgh

    P= FxA ?

    3. The attempt at a solution

    From attached images:

    Mass of Water = 7500kg

    Finding Distance:
    d = tan 18 x 1 = 32.5cm

    Finding mass of displaced water:
    d x h x w = .325 x 1 x .75 = 243.75kg
    but we are dealing with triangle so
    mass of displaced water =243.75/2 = 121.875kg of water displaced

    Calculating Height of water when flap is extended:
    Mass of water = length of tank x width of tank x height of tank
    7.5 = (10-.1625) x .75 x h
    h = 1.0165 cm

    Calculating Force:
    d = 32.5/2 = 16.25 (as we are dealing with triangle not rectangle)

    F x 16.25 = 7500x9.81x1.0165

    F = 4602.4kN

    My Thoughts
    Very sure I am missing something fundamental with this solution but not sure what!!

    I feel I have ignored the fact that the force is being applied at 1.25m not 1m but dont know how to incorporate this.

    Im also unsure if I have used the wrong figures for my Fxd=mgh solution.

    I feel P=F/A can be incorporated but not sure how?

    Thanks for any feedback!

    Attached Files:

  2. jcsd
  3. Nov 16, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi Whatamiat! :smile:

    Are you using the work-energy theorem? (it would have been clearer if you'd said so)

    If so, in the formula work done = force "dot" displacement, the displacement is the displacement of the point of application of the force. :wink:
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