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Bouyant force help

  1. Mar 19, 2013 #1
    Can someone please help me understand this conceptually? If there is an object completely submerged in say water and if there is an apparent weight I don't understand why that object wouldn't accelerate downward. In the air an object can reach terminal velocity and move an constant velocity but I don't understand how the object in water can just stay still. Does that make sense? Thanks.
     
  2. jcsd
  3. Mar 19, 2013 #2
    In air, an object can also stand still. Balloons do. Same principle.
     
  4. Mar 19, 2013 #3
    In a few of my questions they talk about apparent weight in water (when the object is completely submerged) where the buoyant force is smaller than the weight of the object, my question is why doesn't the object accelerate downward in the water. I understand that when the object is partially submerged the weight equals the buoyant force so the net force is zero and the object has no acceleration. But i get confused when it is entirely submerged. If someone could explain to me what I'm missing here I would appreciate it.
     
  5. Mar 19, 2013 #4

    Doc Al

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    I don't think you're missing anything. If the buoyant force is smaller than the weight, and those are the only forces acting, then the object will accelerate downward. But often the object is suspended (by a rope, say) or held up in some manner, so other forces act.
     
  6. Mar 19, 2013 #5
    Just the fact that an object is submerged does not mean that its buoyant force is smaller than its weight. It could have been made submerged by some other force that has since been removed. The motion will depend on the CURRENT forces.
     
  7. Mar 19, 2013 #6

    SteamKing

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    Submarines are good at staying completely submerged without accelerating downward.

    In all floating bodies, including some submerged ones, the weight (W) of the body is balanced by the buoyant force (B). Since Sum F = B - W = 0 = ma, then the body undergoes no motion in the vertical direction.

    If, on the other hand, the body is not floating, this implies that the weight is greater than the buoyancy. In this case,
    Sum F = B-W = ma, but Sum F < 0,
    and the body will sink, accelerating until it reaches some terminal velocity (provided the water is deep enough to permit this).
     
  8. Mar 20, 2013 #7
    Thank you so much everyone. So is it correct to say the only way an object will stay submerged in a liquid and not sink to the bottom is if it has the same density as the liquid?
     
  9. Mar 20, 2013 #8
    Even in this case it might sink - if its initial velocity is downward. If the velocity has no vertical component, then it will stay at some constant depth.
     
  10. Mar 20, 2013 #9

    Doc Al

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    Yes. See: Neutral buoyancy
     
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