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Homework Help: Why would an object become lighter under water?

  1. Nov 27, 2009 #1
    If you suspend a large object beneath surface of a lake....it seems to weigh less than if it was above...why is this? Would it become even lighter if you kept lowering it into the water?
  2. jcsd
  3. Nov 27, 2009 #2


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    Gold Member

    Re: Buoyancy

    The object's weight is offset by the weight of water it displaces. If the object is 1 cubic metre, it will displace 1000kg of water. That 1000kg of water will try to push its way back into the space occupied by the object. In doing so, it will push up on the object with a force of 1000kg.

    An object of sufficient rigidity will not get lighter if it is pushged deeper below the surface since it will displace no more than 1000kg of water at any depth.

    Unless this is homework.

    In which case: what are your thoughts?
  4. Nov 27, 2009 #3


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    Homework Helper

    Re: Buoyancy

    Why is a very good question.
    The simple answer is that there is an upward force equal to the weight of the water displaced - but that doesn't really explain why!
    The important thing to understand is that under the water the pressure acts in all directions.
    If you had a cube of water under the surface there is a force upward exaclty equal to the weight of the cube of water.
    The water doesn't move so the force up must be equal to the weight of this cube.
    No remove the cube and replace it with another cube of another material.
    The force upward doesn't change - so the new cube has an upthrust equal to the weight of the cube of water

    No - assuming the density of the water doesn't change
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