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Homework Help: Pressure? Resultant force?

  1. Aug 13, 2009 #1
    1. The problem statement, all variables and given/known data
    Figure shows cross sections of three hollow objects two of which have openings closed with a soft rubber membrane R ,and the other without an opening just floating inside three bottles of water labelled A,B and C.(sorry I've missed the labels in the diagram.A,B and C are the bottles starting from left to right)
    When an excess pressure is applied to the air above the water surface in all three bottles by pressing their mouths with hands as shown,what change can be observed in the positions of these objects?

    http://img15.imageshack.us/img15/2817/35445.png" [Broken]

    2. Relevant equations

    3. The attempt at a solution

    At first,I thought all three objects would move down.and object B would move down more than object A and C , but the answer says,
    Object A and Object B moves down and object C remains stationary.

    I think I understand why object C remains stationary cause (I'm not sure but)I guess the excess pressure gets uniformly distributed throughout the liquid and since the object is symmetrical,the force acting on it on all sides would be equal and cancel off ,so the resultant force acting on C would be zero?

    But if that were true shouldn't A move sideways? cause the force acting on the membrane would be less and so the resultant force acting on A would force the object to move to the right,maybe?

    Please explain to me where I'm going wrong.

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Aug 13, 2009 #2


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    Initially, the objects float because the buoyant force on them matches their weights and the net force is zero.

    When pressure is applied and the membrane is pushed inward, objects A and B displace less fluid than before, therefore the buoyant force decreases and they sink. Object C is rigid and displaces the same amount of fluid no matter what the pressure is. Therefore the buoyant force on C is the same as before so it keeps on floating.

    All sideways forces cancel because they are equal and opposite at opposing sides at the same depth.
  4. Aug 13, 2009 #3
    I understand now.
    Thank you very much!
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