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Weight of system: water container and rock

  1. Jul 13, 2012 #1
    1. The problem statement, all variables and given/known data

    Container filled with water is placed on a scale.

    case #1 Lid is closed. Rock is placed on top of the lid.

    case #2 Rock is placed inside the water container, it sinks to bottom. Lid is closed.

    How does the weight (reading on the scale) compare in the two cases?

    2. Relevant equations

    3. The attempt at a solution

    I think the weight is the same in the two cases.

    But I have done some tricky questions involving buoyancy/weight.water level, and sometimes the answer is not so simple. Once I can confirm this answer, I will post some variants of this situation and try to see where my confusion is.
     
  2. jcsd
  3. Jul 13, 2012 #2

    Doc Al

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    Staff: Mentor

    You are correct. Any buoyant forces acting on the rock when submerged are internal to the system and will not affect its scale weight.
     
  4. Jul 13, 2012 #3
    1. The problem statement, all variables and given/known data

    Now what if the rock is suspended from a force meter, so that it is under the surface of the water but not on the bottom

    (and the string from the force meter goes through a small hole in the lid, so that mass is still there)

    2. Relevant equations

    3. The attempt at a solution

    I think here the weight of the system becomes

    W = (Weight of water & container) + Buoyancy force on rock

    So the weight on the scale should be less than the two cases before
     
  5. Jul 14, 2012 #4

    Doc Al

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    Staff: Mentor

    You are correct.
     
  6. Jul 14, 2012 #5
    You are correct and this is easy to demonstrate using kitchen scales ( electronic is best)
    And a force meter. If you lower the stone into the water you will see the force meter reading decrease and the scales reading increase as you described
     
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