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Buoyancy help

  1. Jun 5, 2016 #1
    1. The problem statement, all variables and given/known data
    1a)How does a ball floating 50% in water move when a LARGE amount of oil is added?

    1b) if something denser than water were added, how would the ball move?

    1c) if the exp. was done on the moon with different gravity and no atmosphere, how would the ball move?

    2. Relevant equations
    Fb= PA

    3. The attempt at a solution
    For question 1a, I believe that the ball would rise because the water would have to exert a greater force when the oil is on top on the ball, since the oil weighs heavier with air.
    For question 1b, I think that the ball would remain at the same level because the denser substance would just sink to the bottom.
    For question 1c, I said that the ball would sink because there is less of a buoyant force on the moon.

    Can anyone tell me if my answers are correct? I'm not sure if I'm thinking of buoyancy in the wrong sense.
  2. jcsd
  3. Jun 5, 2016 #2
    Hi Joe:

    Based on the reasoning in tour answers, I think you have forgotten an important concept.

    When the ball floats on a fluid, like water or oil, what is the relationship between
    a. the mass of the ball,
    b: the mass of the fluid which the ball has displaced, that is the mass of the fluid that would fill the volume of the ball that which is below the level of the fluid?

    Hope this helps.

  4. Jun 5, 2016 #3
    @Buzz Bloom

    Wait, why would the mass of the ball change? Because the fluid isn't being inserted inside of the ball.
  5. Jun 5, 2016 #4
    Hi Joe:

    I apologize for not being clearer. I was not assuming the mass of the ball changed. I assume that the volume of the ball below the surface plane of the fluid changes if the density of the fluid changes.

  6. Jun 6, 2016 #5


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    Gold Member

    Is the ball more or less dense than the oil? (You have enough information to answer that.)
    Relative to the ground or relative to the water?
    But is the weight of the ball the same?
    Also, it is not clear whether c is in respect of doing a or b on the moon. Likely it is both, so might require two answers.
  7. Jun 6, 2016 #6


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    I interpret part c differently -- as asking what happens if we take the original ball in the original water, remove the atmosphere and drop gravity by a factor of six.
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