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Homework Help: Conservation of Mechanical Energy

  1. Apr 28, 2008 #1
    1. The problem statement, all variables and given/known data
    A massless string is wrapped around a solid cylinder as shown in the diagram. A block of mass 2kg hangs from a string. When released, the block falls a distance of 82 cm in 2.0 seconds. Calculate the mass using the conservation of mechanical energy.


    2. Relevant equations
    d=speed(i) + 1/2at^2
    a=.205 m/s

    V(f)=v(i) + at
    v(f)=.41 m/s^2

    PE=KE(block) + KE(cylinder)
    mgy=1/2mv^2 + 1/2(1/2MR^2)(v/r)^2


    The problem is that I do not know how to find R? Please help!










    3. The attempt at a solution
     
  2. jcsd
  3. Apr 28, 2008 #2

    alphysicist

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    Hi Lma12684,

    I think you neglected the 1/2 in your equation when you calculated a.

    For the value of R, what do R and r represent in your final equation?
     
  4. Apr 28, 2008 #3
    My apologies, they represent the radius. I didn't mean to use different symbols.
     
  5. Apr 28, 2008 #4

    alphysicist

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    So you can see now why you do not need to know the radius?
     
  6. Apr 28, 2008 #5
    Because they cancel??
     
  7. Apr 28, 2008 #6

    alphysicist

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    That looks right; once you correct a and vf I think you can solve the energy equation.
     
  8. Apr 28, 2008 #7
    Thank You~!!!
     
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