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Homework Help: Weight and bouyancy

  1. Dec 5, 2008 #1
    1. The problem statement, all variables and given/known data
    A block of wood floats in a beaker of water. According to Archimedes' principle, the block experiences, an upward bouyant force. If the beaker wit the water and floating block were weighted, would the measured weight be less than the sum of the weights of the individual components? Explain.

    2. Relevant equations

    3. The attempt at a solution

    So here is what I am thinking, I think the weight would be less. If weight is mg but there is a negative force acting through the bouyancy force this should be subtracted, making it less than the sum of the individuals. If my thinking right? Its the weekend and I didn't want to wait until monday :)

    Thanks for any help!
  2. jcsd
  3. Dec 5, 2008 #2


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    What if you had a bucket full of rice? Do you think it will weigh differently if you place a block on top of the rice or bury it in the rice or weight the bucket and the block separately?
  4. Dec 5, 2008 #3
    I suppose then it wouldn't matter at all. So then even though there is a force pushing the block up keeping it a float, the mass of the block is still being pushed down making the whole thing weight the same as the sum of the individuals..
  5. Dec 5, 2008 #4


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    The reason the block floats is a property of the matter - density. But the total matter is not determined by the density.

    The total matter does however determine the weight. Since you are not creating or destroying matter between the two weighings ...
  6. Dec 5, 2008 #5
    makes sense, I suppose I was trying to think too much about the question tricking me. Thank you for the help.
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