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Homework Help: Container of Liquid Accelerated Upward

  1. Jan 15, 2017 #1
    1. The problem statement, all variables and given/known data

    A block floats partially submerged in a container of liquid. When the entire container is accelerated upward, which of the following happens? Assume that both the liquid and the block are incompressible.

    A) The block descends down lower into the liquid.
    B) The block does not ascend or descend in the liquid.
    2. Relevant equations

    ρ = M/(Volume)
    W = Mg
    F = ma
    Archimedes's principle

    3. The attempt at a solution

    I figured that when the block floats partially submerged and is not accelerating, the buoyant force on the block is equal to the block's weight.

    When the block is accelerated upward, then there must be a net upward force on the block; since there are only two forces acting on the block, buoyant force and weight, the buoyant force on the block must increase. Since the buoyant force increases, more of the block's volume must be submerged in the liquid since, by Archimedes's principle, the buoyant force is equal to the weight of the displaced liquid. More liquid should be displaced now, so the block must descend. I chose answer A.

    The correct answer, however, is B.
    What is wrong with my reasoning?
  2. jcsd
  3. Jan 15, 2017 #2
    When a system accelerates it's just as if you have increased gravity ...Remember astronauts in a rocket taking off from earth ..they experience 5 or 6 g because the rocket is accelerating at 4or 5 g ...inside the rocket it feels the same as it would if the earth's gravity had just magically increased.

    So the question is... is the way the block floats dependent on the gravity it experiences , On the moon would it float the same as on earth ?
    Last edited: Jan 15, 2017
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