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Homework Help: Buoyancy of ship, volume displaced, and tension of crane (in/out) of water

  1. Dec 16, 2009 #1
    A crane lifts the 18,000kg steel hull of a ship out of the water.
    The density of steel is known to be 7.8 x 103 kg/m3, while that of water is 1000 kg/m3.

    A) While the steel hull is fully submerged in the water, what is the volume of water displaced by the hull? I really have no idea how to work this problem out.

    Buoyancy = m* density * V displaced * G

    B) What is the tension in the crane's cable when the hull is fully submerged?
    I believe this one is related to A

    Density * Volume * G = F m=(density) Volume
    1000* 18000/10 *10

    f=1.8 x 107

    C) What is the tension in the crane's cable when the hull is out of the water?

    Ft= mg
    Ft = 18,000 * 10 (class standard)
    Ft= 180,000N
  2. jcsd
  3. Dec 16, 2009 #2
    If you have the mass of the material and its density, can't you find its volume? That volume displace the same volume of water.
  4. Dec 16, 2009 #3


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    Gold Member

    Only if the hull fills with water. If it does not, it displaces far more water than its own volume.

    The question seems a bit tricky. It talks about a fully-submerged the hull, which, to me, strongly suggests that the hull has filled with water (unless it's a watertight submarine hull I suppose :tongue2:).

    So, I guess you're right. They're simply talking about the volume of the steel itself, not any kind of bouyant hull-shape.
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