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Homework Help: F=MA but including Bouyancy

  1. Dec 20, 2008 #1
    1. The problem statement, all variables and given/known data

    Before you submerge a block (25N) in water, you place the container of water on a scale. The scale reads 100N.

    You then immerse the block in the water, the block is also hung on a scale that reads 17N. Is the scale reading greater than, less than, or equal to 100N? ( Assume that the object does not touch the sides or bottom of the container at all.)


    2. Relevant equations


    3. The attempt at a solution

    I believe the scale will read more even though if you do a Free Body Diagram of the Block its:

    Bouyant Force + Tension of Scale - Weight = 0
    Bouyant force = Weight - Tension
    Bouyant force = 25N - 17N = 8N UpWard

    So the Weight of the block is canceled out by the Bouyant Force + Tension. However if you think of it intuitively, if the tension of the block is not fully supporting the block itself the bouyant force will take into account weighing down the scale at the bottom therefore it will be Greater than the 100N originally. But maybe im wrong can someone help me out this is very tricky =) Thanks again
  2. jcsd
  3. Dec 20, 2008 #2


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    Homework Helper

    You seem to have it.

    The block displaces water. 8N worth of water. It's as though there is an additional 8N worth of water in the box isn't it? So ...
  4. Dec 20, 2008 #3
    so i am right!? awesome it does weigh more because the water displaced is 8N worth also right?
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