Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B About buoyancy and 2 equations

  1. Jan 26, 2017 #1
    1) My teacher says that the apparant weight of an object in water (floating,sunken,submerged etc) is equal to it's actual weight-buoyant force acting on it.

    That is, wt (ap)=wt (ac)- F(b)

    Where wt (ap) = apparant weight,
    weight (ac)= actual weight.
    And f (b) = buoyant force

    2) But..., if an object is floating or is submerged under water, it's weight is completely supported by the buoyant force acting on it. So shouldn't the buoyant force be equal to the apparant weight of the floating object.?
    i.e., f (b)=wt (ap)

    The equations in 1) and 2) contradict each other.

    Please tell me where I am wrong.
     
  2. jcsd
  3. Jan 26, 2017 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    What about a stone submerged under water? Is it fully supported by buoyancy?
     
  4. Jan 26, 2017 #3
    A submerged object (resting on the bottom) is not completely supported by the buoyancy force , some support comes from the sea bottom , and this support from the sea bottom is the 'apparent weight' ...
    If floating there is no apparent weight ...
     
  5. Jan 26, 2017 #4

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    An object submerged in water will sink to the bottom if its actual weight is more than the buoyant force.

    If it floats below the surface but doesn't sink, then the weight and buoyant forces exactly balance.

    If the weight is less than the buoyant force, it floats to the surface and only part of the object will be submerged, like a boat.
     
  6. Jan 26, 2017 #5

    Saw

    User Avatar
    Gold Member

    Just to complete the answers with specific reference to where you went wrong:

    Right.

    No, in this case the buoyant force is equal to the actual weight. That is why the object is in equilibrium and its apparent weight is zero. But apparent.. for whom? For you, for example: you don't need to make any effort to avoid that the object sinks and if you put a balance scale under the object it will read zero, just as if it were supported by a rope from a hook. But for the water itself, things are different: the water has suffered the actual weight (well, to be accurate, the water has suffered contact force equal to the object's actual weight and has reacted by applying another contact force of the same magnitude, i.e. the buoyant force).
     
  7. Jan 27, 2017 #6
    I knew there was something off about the submerged part.
     
  8. Jan 28, 2017 #7
    Restating a comment from above, if an object is completely submerged and is not moving vertically up or down then it is in equilibrium, the buoyant force equals the actual weight. The apparent weight is zero.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: About buoyancy and 2 equations
  1. Question about buoyancy (Replies: 48)

Loading...