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Tension in the string and tangential accleration

  1. Aug 31, 2015 #1
    1. The problem statement, all variables and given/known data
    A particle of mass m is whirled on the end of a string of length R. The motion is in vertical plane in Earth's gravitational field . The instantaneous speed is v when the string makes angle Ө with the horizontal. Find the tension T in the string and the tangential acceleration at this instant.

    2. Relevant equations


    3. The attempt at a solution
    This is my classical mechanics homework problem.
    T= Fc+W sin theta .
    I have come up with this equation , is this correct . And how to find the tangential acceleration?
     
  2. jcsd
  3. Aug 31, 2015 #2

    mfb

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    How did you come up with it, and what are Fc and W? The final answer should use R, v and Ө only.
     
  4. Aug 31, 2015 #3
    Can you show me how ?
     
  5. Sep 1, 2015 #4

    haruspex

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    m and g also.
     
  6. Sep 1, 2015 #5

    haruspex

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    Express Fc in terms of m, g, r, v, Ө (you won't need all of them). There's a standard equation you should use (and which should have been listed in the Relevant Equations).
    Express W similarly.
    Be careful with signs. Define whether up or down is considered positive, and whether the value of g is taken to be positive or negative. (E.g. if up is positive, then you can write gravitational acceleration as g, taking a negative value, or as -g, taking a positive value.)
     
  7. Sep 1, 2015 #6

    mfb

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    It's not just signs, you should consider both contributions as vectors, not with their absolute values.
    Right, of course.
     
  8. Sep 1, 2015 #7
    I am not getting it :(
     
  9. Sep 1, 2015 #8

    mfb

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    Then start with the hints haruspex gave.
    At least write down the equations you know that could be relevant here, and see if one of those has quantities that are given here.
     
  10. Sep 1, 2015 #9
    And what is that relevant equation from which I should start my assumption?
     
  11. Sep 1, 2015 #10

    haruspex

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    The formula for centripetal acceleration.
     
  12. Sep 1, 2015 #11
    mv^2/r , this one? So I should substitute value of Fc in T= Fc+W sin theta? I am sorry , if my silly questions are annoying you :cry:
     
  13. Sep 1, 2015 #12

    haruspex

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    yes. Substitute for W too (mg).
     
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