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Tension in a string along with a Pseudo force

  1. Apr 23, 2007 #1
    1. The problem statement, all variables and given/known data

    A block of mass 50g is suspended from the ceiling of an elevator.Find the tension in the string if the elevator goes up with an acceleration of [tex]1.2m/s^2[/tex]

    2. Relevant equations

    [tex]F_{net}=ma_{net}[/tex]

    3. The attempt at a solution
    I have reached here:...but with this i dont get the books answer...The book takes net acceleration to be 0:eek:
    If F be the pseudo force...then,
    [tex]T-F_g-F=ma[/tex]
    [tex]a=1.2[/tex]



    So solving this i get an answer that differs from the one given in a problem book...that is 55N(the book's ans)

    Ive realized that we get 55N if we take [tex]a_{net}=0[/tex].Now how is this possible?:confused:
     
    Last edited: Apr 23, 2007
  2. jcsd
  3. Apr 23, 2007 #2

    andrevdh

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    Homework Helper

    The real forces that the mass experiences are the tension in the string, T, and its weight, W. The resultant of these two forces are responsible for its given acceleration. So there is no need for a pseudo force.

    Also with a mass of 50 grams the tension comes to 0.55 N. A mass of 5.0 kg will give 55 N.
     
    Last edited: Apr 23, 2007
  4. Apr 23, 2007 #3
    Well...i dont seem to get the point that u have refered to...because...we certainly need a pseudo force.But i understand that relative acceleration will be zero.
     
  5. Apr 23, 2007 #4

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Depends upon your point of view. I would say there are two "forces" on the mass: one of mg= .05kg (9.8/s2)= 0.49 N downward and the "pseudo" force of ma= 0.5kg(1.2 m/s2)= 0.06 N, also downward, so that the total "force" on the spring and its tension is 0.06+ 0.49= 0.55 N.

    Of course, you could also have said that there is a net "acceleration" of 11 m/s2 downward given F= ma= 0.55N.
     
  6. Apr 23, 2007 #5
    Thnx for the solution to the problem
     
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