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True/False -ish question, buoyancy

  1. Nov 23, 2013 #1
    1. The problem statement, all variables and given/known data

    For each statement, select the phrase that makes the statement true (e.g., if the answer to the first is A and to the others B, enter ABBBB):

    A) increases B) decreases C) less than

    D) greater than E) stays the same F) the same as

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

    Two balloons of the same volume are filled with helium and air, respectively. The buoyant force on the helium-filled balloon is ______ the buoyant force on the air-filled balloon.

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

    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 upward.

    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 duck is ______ the reading on the other scale.

    2. Relevant equations


    3. The attempt at a solution

    My guess was AFBAA

    I am guessing my error is somewhere in the last two.

    - I figured when a bucket was in an elevator accelerating upwards, the gravatational force would be pushing down on the object in the water forcing the water level to raise

    -For the last one, my assumption was that the combined volume of the duck and the water would make it heavier compared to the one without a duck.

    Any help please? Are my first 3 correct?
  2. jcsd
  3. Nov 23, 2013 #2
    What is your reason for your answer, try to explain it. That way you may also realise, if one is wrong. For instance your 2nd answer is correct, because the balloons are of same size, and the buoyant force is exerted by the air around the baloons on the balloons, thus it must be the same for equally sized balloons.

    Now try to explain your first and third answer. I think you should realise then, that one of them doesn't make sense, if the other is correct.
  4. Nov 24, 2013 #3
    Sorry for the late reply- hope you still see this;

    reasoning for 1 - the overall buoyancy force on the boat pushing down is heavier with the cement block in it thus a higher water level. Once the cement is thrown off the boat, the weight decreases making the water level decrease. Just realized that it should have been decrease, not increase. Haha

    For 3 - I am second guessing my choice on this one, because the cement was thrown into the water, I think it would then increase, so I think the water level may stay the same?
  5. Nov 24, 2013 #4
    Yes, that seems more like it.
  6. Nov 24, 2013 #5
    Hmm, so then are my last two correct?
  7. Nov 24, 2013 #6
    The one with the elevator is bugging me a bit... however, I would think your answer is correct (at least I would say the same). But I am not entirely sure.

    The one with the duck seems wrong. Not only does A) not fit into the sentence, also I believe a duck has a lower density than water.
  8. Nov 24, 2013 #7
    Oh wow, actually the A for the duck was actually supposed to be F, the same. But yeah that will still be incorrect. So my assumption now would be; since it is touching the bottom and not floating, wouldn't it be greater then the one without the duck??
  9. Nov 25, 2013 #8
    I guess one should assume, the duck floats, and thus only its legs are in the water. They could have a higher or lower density, if much of the duck is above the surface, it could still weight more, although I would say this depends on the size of the duck as well. Nonetheless, interesting questions :-)
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