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Tension in a Pendulum String - Oh my!

  1. Oct 5, 2013 #1
    So I have one attempt left and want to make sure my answer is correct, so double checking me would be super awesome!

    You are given a pendulum composed of a 0.030 kg mass on the end of a 0.42 m long massless string. The pendulum is moved 25° from the vertical and released. Find the tension in the string when the mass is at the release point,
    θ = 25°.

    Tsin(65)-mg=0
    Tsin(65)=mg
    T=.32 N

    This is one solution I came up with.

    The other option I am unsure of is if the T force would be 0 because "there is no net force directed along the axis that is perpendicular to the motion. Since the motion of the object is momentarily paused, there is no need for a centripetal force." -http://www.physicsclassroom.com/class/waves/u10l0c.cfm

    Thanks for the guidance.
     
  2. jcsd
  3. Oct 5, 2013 #2
    The centripetal force is indeed zero in the extreme position. However this does not imply T=0.
    The net force along the radial direction should be zero. And this net force includes gravity's contribution as well.

    The first "option" is not good because the body's acceleration along the vertical direction is not zero.
     
  4. Oct 5, 2013 #3
    So,
    Fradial= -T + mgsin(65)
    T=mgsin(65)
    T=.27 N
     
  5. Oct 6, 2013 #4
    So I have one attempt left and want to make sure my answer is correct, so double checking me would be super awesome!

    You are given a pendulum composed of a 0.030 kg mass on the end of a 0.42 m long massless string. The pendulum is moved 25° from the vertical and released. Find the tension in the string when the mass is at the release point,
    θ = 25°.


    "The centripetal force is indeed zero in the extreme position. However this does not imply T=0.
    The net force along the radial direction should be zero. And this net force includes gravity's contribution as well."--nasu




    So,
    Fradial= -T + mgsin(65)
    T=mgsin(65)
    T=.27 N
     
  6. Oct 6, 2013 #5

    ehild

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    Homework Helper
    Gold Member

    It is correct.

    ehild
     
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