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Implementing angular momentum approach in problem

  1. May 8, 2016 #1
    1. The problem statement, all variables and given/known data
    An object is in uniform circular horizontal motion at the end of a chord of length L. Its tangential speed is v. The chord is pulled in to length 0.5L in such a way that the tension in the chord remains constant. As a result, the tangential speed:

    a) remains constant
    b) increases to 2v
    c) decreases to 0.5v
    d) increases to 1.4v
    e) decreases to 0.7v

    2. Relevant equations
    The correct answer is E.
    This can be solved using T = m * v^2/r approach. I get this, and I know how this works.

    3. The attempt at a solution
    Here's what I though initially: we do not have any net torques on the system [using an analogy of planetary motion around the sun, where angular momentum is conserved, substituting Tension force for gravitational force], so angular momentum has to be conserved.

    Thus:

    Li = Lf
    [m*v*r]i = [m*v*r]f

    I eliminate the m's, and then plug:
    v*L = x * 0.5 L
    thus x = 2v, which is b, an incorrect answer.

    Could anyone please explain why this approach is incorrect?

    Thanks in advance for the assistance!
     
  2. jcsd
  3. May 8, 2016 #2

    PeroK

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    If the tension in the chord doesn't change, why would the object move in?

    Try to imagine actually carrying out this experiment.
     
  4. May 8, 2016 #3
    Object moves in because it slows down?
     
  5. May 8, 2016 #4

    PeroK

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    So, someone would have to slow it down in this experiment. If they didn't, the tension would necessarily increase.

    This is another slighly disengenuous question, where you are being caught out by what is not said in the question!
     
  6. May 8, 2016 #5
    The authors write really crummy questions. I have been dealing with them for the past 1.5 days now :-/
     
  7. May 8, 2016 #6

    PeroK

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    Personally, I don't like this sort of question. But, perhaps it's good not to assume that everything is in the question. Or, at least to analyse the question more. My initial thought was: that's not right, the tension must increase to pull the object in. Then I realised what was intended.
     
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