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Buoyancy problem help

  1. Nov 29, 2006 #1
    Fill in the blanks with:
    A) increases B)decreases C) less than D) greater than E) stays the same F)stays the same as

    i) A block of wood floats in a pail of water in a stationary elevator. The level of water in the pail _____ when the elevator accelerates downward.

    ii) Two identical shallow containers are placed on identical scales. A live duck is put in one container. Both containers are then filled with water to their brims; it is observed that the duck is standing on the bottom of its container. The reading on the scale under the container with the ducks is _____ the reading on the other scale.

    iii) A boat containing a cement block floats in a swimming pool. The water level in the pool ___ when the block is thrown to shore.

    iv) A boat containing a cement block floats in a swimming pool. The water level in the pool ___ when the block is dropped into the water.

    v) Two balloons of the same volume are fulled with helium and hydroge, respectivley. The buoyant force on the helium filled balloon is ______ the buoyant force on the hydrogen filled balloon.


    I said ADABC but that is not correct. I am fairly sure of iii, iv, and v.
    ii i am not sure what aspects to consider, and for i I cannot decide if it would stay the same or rise.
    Thanks for any help!
     
  2. jcsd
  3. Nov 29, 2006 #2

    OlderDan

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    Check the red ones. On a first read, I think they all have different answers from the ones you chose and from each other.
     
  4. Nov 29, 2006 #3
    hmm...
    For iii) i am thinking B
    for v) hydrogen is less dense than helium so wudnt it push up with a greater force? im kinda confused about that one.
    for i) i really cannot seem to get my reasoning straight, wouldnt it have a greater weight if it was going downwards, displacing more water? But on the other hand its density relative to water wouldn't change...
     
  5. Nov 29, 2006 #4

    OlderDan

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    For iii) when the block is in the boat it is displacing its own weight of water. When it is thrown out, that water is no longer dispalaced. Less displaced water means lower level. I think you've got it

    For v) you are being asked for the buoyant force. What determines the buoyant force?

    For i) how does water pressure increase with depth? Neglecting evaporation, what would happen if you took the pail and block to the moon? How does that compare to the situation in the elevator. You have the key in your quote.
     
  6. Nov 29, 2006 #5
    ok so for i) i am now thinking E
    and for v) i am thinking d
     
  7. Nov 29, 2006 #6
    does that sound ok?
     
  8. Nov 29, 2006 #7
    although research on the net seems to indicate that helium is less buoyant than hydrogen
     
  9. Nov 29, 2006 #8
    Just got the right answer.
    It was EDBBF
    i forgot the exact definition of buoyant force.
    Thank you so much!!
     
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