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Quick Q re: density and buoyancy

  1. Jan 23, 2016 #1
    1. The problem statement, all variables and given/known data
    We fully submerge an irregular 3 kg lump of material in a certain fluid. The fluid that would have been in the space now occupied by the lump has a mass of 2 kg. a) When we release the lump, does it move upward, downward, or remain in place? b) If we next fully submerge it in less dense fluid and again release it, what does it do?

    2. Relevant equations

    3. The attempt at a solution
    For a) I just assumed that Fg is > b since the mass of the lump is greater than the mass of the water displaced.
    I know that for b) it also goes downward, but why? Does density simply not matter because it's not factored into the equation for buoyancy or would the result be different if we submerged the lump in a fluid that is more dense?
  2. jcsd
  3. Jan 23, 2016 #2

    Suraj M

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    What are the factors buoyant force is dependent on ? and what factors remain constant when when we use a less or more dense liquid instead of the one we are using now?
    Last edited: Jan 23, 2016
  4. Jan 23, 2016 #3
    Well, Fb is dependent on mass of the fluid and g. Mass=ρV.. So as density of the fluid increases, so does its mass and therefore, its buoyancy? But if it decreases, the mass decreases as well so Fg will be even greater than Fb and the object will definitely go downward...? Is this correct?

    Btw, thanks for the help!
  5. Jan 23, 2016 #4


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    Yes. What is the borderline case?
  6. Jan 23, 2016 #5

    Suraj M

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    Your answer is right, I was just responding to this

    Hope you got your answer.
  7. Jan 24, 2016 #6
    Hi haruspex! Sorry, I'm not sure that I understand. Borderline case as in...?
  8. Jan 24, 2016 #7
    I believe so, thanks for your help! Sometimes I feel completely certain I know how to solve a problem in physics only to be totally wrong so I appreciate your confirmation that I was going in the right direction :)
  9. Jan 24, 2016 #8

    Suraj M

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    You considered a case where density is more than or less than then that leaves out a borderline case right? Something in between?
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