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Confusion of circle and sphere for physics problems

  1. Jun 28, 2012 #1
    1. The problem statement, all variables and given/known data

    The problem is attached

    2. Relevant equations

    P=F/A

    3. The attempt at a solution

    I did the question like this (got wrong answer though):

    Surface area of sphere=4∏r2=4×∏×0.252
    Atmospheric pressure=1.01×105

    Force=1.01×105×4×∏×0.252≈80000N
    Actual answer 20000N

    So my friend said something about using a circle instead? I tried it and indeed got the right answer. So why do we use a circle's area instead of sphere.
     

    Attached Files:

    Last edited: Jun 28, 2012
  2. jcsd
  3. Jun 28, 2012 #2
    Check the value of radius.
     
  4. Jun 28, 2012 #3
    i Changed the radius but the answer still reamins the same as i did the question previously itself
     
  5. Jun 28, 2012 #4
    Pressure is the force per unit area applied in a direction perpendicular to the surface of an object.

    The force is not completely perpendicular to the whole surface of hemisphere.
     
  6. Jun 28, 2012 #5
    You have to resolve the pressure force acting on each element of area of one of the hemispheres into the component in the direction that the rope is pulling. This is equivalent to using the projected area of the hemisphere times the atmospheric pressure.
     
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