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Pendulum help

  1. Oct 14, 2013 #1
    1. The problem statement, all variables and given/known data
    You are given a pendulum composed of a 0.030 kg mass on the end of a 0.60 m long massless string. If the pendulum is moved 30° from the vertical and given an initial speed of 0.39 m/s tangent to the support string and away from the vertical, how much higher relative to the release point of the pendulum swing will the pendulum rise?


    2. Relevant equations
    KE=1/2mv^2
    PE=mgL(1-cosθ)


    3. The attempt at a solution

    Here is what I did. I found the velocity at the bottom of the swing.
    1/2vf^2 = 1/2vi^2 + mgL(1-cosθ)
    and I got vf= 1.3 m/s
    Then I did the same thing again, except I set vi to 1.3 and vf = 0
    1/2vi^2 = mgL(1-cosθ)
    and find θ, which I got 31.1 then I subtracted Lcos30 by Lcos31.1 and got .005 m. The answer is incorrect, can anyone help? BTW I already cancelled out the mass in all the above work.
     
  2. jcsd
  3. Oct 14, 2013 #2

    arildno

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    It seems you forgot the m's in your kinetic energies??
     
  4. Oct 14, 2013 #3
    Hi 386221

    Welcome to PF!!!

    Your work looks good to me .What is the correct answer ?
     
  5. Oct 14, 2013 #4

    arildno

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    I haven't checked the data, Tanya Sharma, but it seems that OP is using 1/2v^2, rather than 1/2mv^2.
    I agree that OP, apart from that seems to have chosen a correct, if somewhat overcomplicated solution since there is no need to use the intermediate position at the bottom.
     
  6. Oct 14, 2013 #5
    check again.
     
  7. Oct 14, 2013 #6
    I checked the data before replying.OP has mentioned that he cancelled the masses in his work,which he/she did.He/She has erroneously written 1/2v^2 instead of 1/2mv^2.
     
  8. Oct 14, 2013 #7

    arildno

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    `Well, then his "incorrect" answer might just mean that he rounded off numbers in an incorrect way in his intermediate step.
    Thank you, Tanya Sharma, for having checked the data! :smile:
     
  9. Oct 14, 2013 #8
    For an easier method just set potential energy at the point of beginning of motion as 0.
    ##U_i+K_i=U_f+K_f##
    ##K_i=U_f##
    ##1/2(mv^2)=mg\Delta h##
     
  10. Oct 14, 2013 #9

    arildno

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    There is a couple of sources for errors which might explain the incorrectness of OP's answer:
    1. He might at some point have made a mix-up of radians versus degrees.
    2. Or, and I think this is the likeliest source for the error error: the arccosine function is typically defined relative to the interval 0 to pi, while in this problem, the answer might have be given for angles in the interval -pi/2 to pi/2 instead. That is, a minus sign might be missing in OP's solution for the angle.
     
  11. Oct 14, 2013 #10
    The angle is slightly off...31.0323136133
    (and there was another error in cos31.1)
     
  12. Oct 14, 2013 #11
    I tried changing the second angle to 31.03 and got delta h of .009 and it was wrong. I also tried using 1/2mv^2=mgh and got .0077 for h and that was wrong as well. I honestly have no clue on how to proceed all my work seems correct...
     
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