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Forces affecting a body at the bottom of water

  1. Oct 20, 2011 #1
    Consider a body at the bottom of water whose density is certainly more than that of water,so it settles under water:
    [PLAIN]http://img710.imageshack.us/img710/1778/unled123copy.jpg [Broken]
    I'm trying to explain how the body is under equilibruim although the buoyancy is less than gravity
    That's because of the normal force of the bottom of the container,or the reaction of the container to the body pushing on it due to its weight,thus we have


    Is that true?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 21, 2011 #2
    Pretty much.
  4. Oct 21, 2011 #3
    What do you mean by "Pretty much" :D ?
    Is this true or not?
  5. Oct 21, 2011 #4
    Is this true or not?[/QUOTE]I think you are correct.
    His pretty much means if the system is in non inertial frame then your equations may(depend on direction of acceleration) wrong. In that case you need to add pseudo forces.
  6. Oct 21, 2011 #5


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    Fb (the buoyancy force?) may be affected by the details of how the object contacts with the bottom of the tank. If there were a lip / seal around the outer bottom edge, not allowing water in underneath, then there might be no buoyancy force (only a small amount of air pressure in the space. The upwards force could be largely from the forces through the lip / seal and not due to hydrostatic pressure (upthrust).
  7. Oct 23, 2011 #6
    Okay,but this is basically not wrong..
    In which case can we say that this equation works?
  8. Oct 23, 2011 #7
    it works.

    The buoyant force is equal to the weight of the volume of water displaced and at the same
    time gravity is pulling the object in the opposite direction.

    So the resultant force holding the object in equilibrium is your normal force....a force less
    than the weight of the object in air,
  9. Oct 24, 2011 #8
    Yeah,Okay thanks very much :)
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