Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Buoyancy concept

  1. Jun 15, 2008 #1
    the problem is:
    a block that weighs W floats exactly with 1/2 of its volume below the waterline. What is the buoyant force?

    Answer is W, but why is it W?

    I thought that its 1/2 W because only 1/2 of the volume is below the waterline. if the buoyant force equals to the downward force, wouldn't the block be floating ON the waterline?
     
  2. jcsd
  3. Jun 15, 2008 #2
    By reading a few other posts on this forum, I think that I understand this to be an equilibrium concept in which if it is floating, then the Fb = to the weight of the block. However, what would the Fb be if the block was floating in the waterline?
     
  4. Jun 15, 2008 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Right.
    What do you mean? If it's floating, and the only forces acting are gravity and the buoyant force--what can you conclude?
     
  5. Jun 15, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    If something is floating, then the buoyant force equals the object's weight. That's that!

    But how much of the object is under water is a different question. That depends on how the density of the object compares to the density of water.
     
  6. Jun 15, 2008 #5
    In other words, is Fb still W if the block floats ON the waterline? (without any volume of the block submerged)
     
  7. Jun 15, 2008 #6

    Doc Al

    User Avatar

    Staff: Mentor

    That's not physically possible. If the object has weight it must displace some water.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook