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Homework Help: Qualitative physics question

  1. Sep 22, 2012 #1
    1. The problem statement, all variables and given/known data

    A beaker of water rests on a scale. A metal ball is then lowered into the beaker using a string tied to the ball. The ball doesn't touch the sides or bottom of the beaker, and no water spills from the beaker.

    Does the reading on the scale increase, decrease, or stay the same?

    2. Relevant equations

    3. The attempt at a solution

    I think it increases, cause your introducing an object ontop of the water and u are therefore chaning the mass of the water

    AM I wrong?
  2. jcsd
  3. Sep 22, 2012 #2
  4. Sep 22, 2012 #3

    Doc Al

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    Staff: Mentor

    Why would the mass of the water change? What does change about the water?
  5. Sep 22, 2012 #4
    is the object just touches the water and is ontop then I dont know, i guess there is no bouyant force, just the ball displaces some fluid from the top but since displaced fluid = to the mass it makes no difference, is that right?
  6. Sep 22, 2012 #5
    Sure there's buoyant force. Have you ever held a rock or something heavy under water? Although it sinks, it still appears lighter. Whenever an object displaces water, there will be a buoyant force on the object. If the metal ball sinks, it's denser than the water, and the mass of displaced fluid will not equal the mass of the ball. Even if the metal ball were the same density, as the water, there would be a buoyant force.

    For example, if the ball weighs 5N, and when it sinks, it displaces 2N of water, the weight of the ball would be 5N-2N=3N underwater. 2N is the buoyant force.

    The answer to this question very much has to do with this buoyant force.
    Last edited: Sep 22, 2012
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