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Finding speed with a pulley?

  1. May 27, 2009 #1
    Finding speed with a pulley???

    1. The problem statement, all variables and given/known data

    Two objects are connected by a light string
    passing over a light frictionless pulley as
    shown in the figure. The 5.9 kg object is
    released from rest at a point 4.53 m above the
    floor.
    The acceleration of gravity is 9.8 m/s2 .

    What is the speed of the each object at
    the instant before the 5.9 kg hits the floor?
    Answer in units of m/s.

    2. Relevant equations



    3. The attempt at a solution
    I use the energy equation where
    PE(initial)=KE(final)
    and solve for v but it does not seem to work
     
  2. jcsd
  3. May 27, 2009 #2
    Re: Finding speed with a pulley???

    decrease in PE = increase in KE
     
  4. May 27, 2009 #3

    tiny-tim

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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi physics10189! Welcome to PF! :wink:
    Show us your full calculations, and then we can see what went wrong, and we'll know how to help! :smile:
     
  5. May 27, 2009 #4
    Re: Finding speed with a pulley???

    well here is what i did

    mgh=.5mv^2
    h=Lcos(theta)
    mass cancel out
    so this is what i came up with
    v=2gLcos(theta)
    where L=10m of the string's length
     
  6. May 27, 2009 #5
    Re: Finding speed with a pulley???

    whoops wrong problem
    scrath the last reply

    ok its the same way kinda
    so here is what i did

    PE(initial)=KE(final)
    mgh=.5mv^2

    masses cancel out
    h=4.53m
    there for
    v=sqrt(2gh)
     
  7. May 27, 2009 #6
    Re: Finding speed with a pulley???

    Is the "final" PE of the system zero?
    See my earlier post.
     
  8. May 27, 2009 #7
    Re: Finding speed with a pulley???

    yes because just before the blocks hit the ground can we assume that the height is zero therefore the PE final is zero???
     
  9. May 28, 2009 #8
    Re: Finding speed with a pulley???

    What answer are you getting?
     
  10. May 28, 2009 #9

    PhanthomJay

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    Gold Member

    Re: Finding speed with a pulley???

    A figure, or description, showing what is theta, the mass of the other object, etc., sure would help.
     
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