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Fluids Archimedes principle apparent weight question

  1. Apr 16, 2010 #1
    The problem statement, all variables and given/known data

    Vessel contains water.It is placed on the pan of a physical balance.A solid(specific gravity=1) is suspended from an thread(from an independent support),such that it is immersed completely in the water.
    What the weight it reads?

    If the thread was tied to the arm of the balance,what would the weight have been?

    The attempt at a solution
    First Part:
    Water exerts a buoyant force on the solid.This results in a reaction force (equal and opposite).
    Thus the weight read is that of vessel with water+solid.

    Correct?
    Second Part:
    My book says it is equal to the weight of the vessel with water only.How it is so?How is it different from the first case?
     
  2. jcsd
  3. Apr 16, 2010 #2
    Although the answer is correct you shouldn't say the weight on the balance would be the weight of water plus that of the solid. Rather it is better to say that the reading shows the weight of water plus the weight of water that would have been enclosed in the solid had it been made of water. Here the density of solid is equal to that of water (a coincidence).
     
  4. Apr 16, 2010 #3
    Okay.But what about the 2nd part.That is more important.Please read that too.
     
  5. Apr 17, 2010 #4
    The weight reading is actually the force needed to balance the torque of the other arm. Since distances are equal from both sides this aspect is not important. But how do you suppose the force on the vessel changes if the thread were tied to the other end?
     
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