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Buoyant Force of a block in water

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  1. Apr 5, 2015 #1
    A flask of water rests on a scale that reads100 N. Then, a small block of unknown material is held completely submerged in the water. The block does not touch any part of the flask, and the person holding the block will not tell you whether the block is being pulled up (keeping it from falling further) or pushed down (keeping it from bobbing back up).

    What is the measurement on the scale now?

    a) Greater than 100 N
    b) Less than 100 N
    c) Equal to 100 N
    d) We cannot tell without more information

    The correct answer is a. Though I don't know how they reached this conclusion
    Could someone please help me with this? :)
     
  2. jcsd
  3. Apr 5, 2015 #2

    mfb

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    2016 Award

    Staff: Mentor

    What is the force the person has to apply for different cases (like no mass, something with the same density of water, something heavier)? The force of the scale plus the force by the person together have to keep the whole system balanced, so knowing one allows to find the other one.
    You'll see that all those cases lead to answer (a).
     
  4. Apr 5, 2015 #3
    To expand on what mfb said, draw a free body diagram for each of the three cases that mfb has identified. The importance of drawing free body diagrams cannot be overemphasized.

    Chet
     
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