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Homework Help: Buoyant force same for all object of same volume?

  1. Oct 28, 2011 #1
    Rank the buoyant forces exerted on the following five objects of equal volume from the largest to the smallest. Assume the objects have been dropped into a swimming pool and allowed to come to mechanical equilibrium. If any buoyant forces are equal, state that in your ranking. (Use only ">" or "=" symbols. Do not include any parentheses around the letters or symbols.)
    (a) a block of solid oak
    (b) an aluminum block
    (c) a beach ball made of thin plastic and inflated with air
    (d) an iron block
    (e) a thin-walled, sealed bottle of water

    My answer? They are all equal since the buoyant force depends only on the volume displaced (whether the buoyant force is stronger or weaker than the weight of the object is irrelevant in this case I think). Cramster also says they are all equal and one of my classmates say it is equal too.

    Webassign says they are not equal.
    Last edited: Oct 28, 2011
  2. jcsd
  3. Oct 28, 2011 #2


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    When they achieve mechanical equilibrium, some of them are not fully submerged. Those not fully submerged, displace an amount of water that's' less than for those that stay submerged.
  4. Oct 28, 2011 #3


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    What's the definition of 'buoyant force' intended in the question?
  5. Oct 29, 2011 #4
    Thanks a lot man. So in the end it comes to e=b=d>a>c
  6. Oct 29, 2011 #5
    But don't forget the restriction that they are at mechanical equilibrium. Buoyancy force is only related to volume of water that is replaced. While in equilibrium, some of them are not completely in water, which means no water displaced.
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