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If the object floats, the buoyant force will be equal to its

  1. Aug 24, 2016 #1
    1. The problem statement, all variables and given/known data

    If an object floats, the buoyant force will be equal to its:

    a. Mass c. Specific Gravity

    b. Density d. Weight

    2. Relevant equations


    3. The attempt at a solution
    Is it weight?
     
  2. jcsd
  3. Aug 24, 2016 #2

    Doc Al

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    What does it mean to float? What must be the net force on a floating object? What forces act on a floating object?

    (Use those questions to describe the reasoning behind your answer.)
     
  4. Aug 24, 2016 #3

    Bystander

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    What are the units?
     
  5. Aug 24, 2016 #4
    I got no units. Nothing. Just wondering when an object floats, is it because the buoyant force is equal to its mass, specific gravity, density or weight?
     
  6. Aug 24, 2016 #5

    Doc Al

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    Well, which of those possible answers has the same units as buoyant force? (That's Bystander's tip!)
     
  7. Aug 24, 2016 #6
    Weight? (Newton)
     
  8. Aug 24, 2016 #7

    Doc Al

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    Right.

    But it sounds like you need to learn more about buoyant force. This particular question can be answered without knowing anything (except units), since only one of the choices is even a force. But they won't all be like that.
     
  9. Aug 24, 2016 #8
    Thanks. I'm just new to physics. I had physics in high school over a decade ago but I don't remember much, even though I did well. I'm taking a class in college now (getting another degree) and I'm just having trouble with simple questions! :/
     
  10. Aug 24, 2016 #9
    Also, a completely submerged body displaces a volume of liquid equal to its weight too, right?
     
  11. Aug 24, 2016 #10

    Doc Al

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    Are you asking if the weight of the liquid displaced equals the weight of the object? Not in general.
     
  12. Aug 24, 2016 #11

    haruspex

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    I think ByStander meant dimensionality, not units. If you know the units that will tell you the dimensionality, but not the other way around.
    Force, mass, density all have different dimensionality. It only makes sense to add items or equate items of the same dimensionality.
    Weight is a force.
     
  13. Aug 24, 2016 #12

    phinds

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    Think of a large Styrofoam beach ball. It would displace an amount of water that was quite heavy. Is the beach ball that heavy?
     
  14. Aug 24, 2016 #13

    haruspex

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    Only if the only other force acting on the body is gravity, i.e. it is floating with neutral buoyancy.
    Otherwise, all you can say is that it displaces its own volume.
    If it is tethered to the bottom then the buoyant force is also having to balance the tension in the tether.
    If it is resting on the bottom then the buoyant force is assisted by the normal force.
     
  15. Aug 24, 2016 #14
    Right, not quite heavy... So, it displaces a volume of liquid equal to its density or buoyant force? Maybe to its own volume?
     
  16. Aug 24, 2016 #15
    Thank you!!
     
  17. Aug 24, 2016 #16

    phinds

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    Which one do you think? How could it be density? Do you understand the use of units in figuring out this kind of thing?
     
  18. Aug 24, 2016 #17

    haruspex

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    But note that was for a completely submerged body. A floating ball of styrofoam will not be completely submerged. What will the displaced volume correspond to in this case?
    One of your answers in post #14 was sort of correct, that the weight of the water displaced is equal to the buoyant force, but that is true whether the object is floating, resting on the bottom, or tethered. In the specific case of a floating body, what other force is the buoyant force equal to?
     
  19. Aug 24, 2016 #18
    Volume. Sorry. I'm not a genius. Had physics over a decade ago and don't remember much.
     
  20. Aug 25, 2016 #19

    phinds

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    OK, well you didn't answer my question about units so let me be specific. NEVER give an answer to a physics question unless you are confident that you have the units right. That catches LOTS of the mistakes that we all make when we are starting out.
     
  21. Aug 25, 2016 #20
    Thanks again...

    In case of a floating object buoyant force equals weight right?
     
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