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Homework Help: A difficult problem with dynamics

  1. Dec 26, 2009 #1
    This is problem from the extension section from my Gr.12 physics text book
    1. The problem statement, all variables and given/known data
    Tarzan (m=100kg) hold one end of an ideal vine (infinitely strong, completely flexible, but having zero mass) The vine runs horizontally to the edge of a cliff, then vertically where Jane (m=50kg) is hanging on,above a river filled with crocodiles. A sudden sleet storm has removed all friction. Assuming that Tarzan hangs on, what is the acceleration toward the cliff edge?


    2. Relevant equations
    Newton's laws of motion.
    Projectile motion.


    3. The attempt at a solution
    Horizontal & vertical distance is not given, making it absolutely difficult to apply projectile applications.
    I know that for Tarzan
    (horizontally) Fnet = Tcosθ
    (vertically) Fnet = Tsinθ - Fg b/c he will be moving on the vine.
    I know that for Jane,
    0=T-Fg, since she is in a stable position
    T=Fg
    I am completely lost on this question, can anybody help me or hint to me something about the solution. :confused:
     
  2. jcsd
  3. Dec 26, 2009 #2

    berkeman

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    Staff: Mentor

    Jane's weight provides the force that accelerates the combined mass of both Tarzan and Jane. Draw free body diagrams for both T and J -- that should help.

    Where the heck is Cheeta anyway?
     
  4. Dec 26, 2009 #3
    Who is Cheeta?
    look at the attached file to see the diagram .. Tarzan is accelerating toward Jane ..
     

    Attached Files:

  5. Dec 26, 2009 #4
    Here is the free body diagram .. yet it can not be used to find the acceleration if tension is not given
     

    Attached Files:

  6. Dec 26, 2009 #5

    berkeman

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    LMAO. Obviously you haven't done your background research on Tarzan:

    http://en.wikipedia.org/wiki/Tarzan

    Cheeta is the chimp who should be trying to save T & J. Jeeze.

    And back on-topic, Tarzan's weight (the mg vector down in the FBD) doesn't matter, since he is on a frictionless surface. All he does is add to the mass that Jane's weight accelerates. The tension in the vine pulls T horizontally to accelerate him. J's weight is what causes the force that accelerates them both as a coupled system.

    Show us a combined drawing of them both, showing the tension force in the vine, J's weight vector, and the accelerating force on T. Use the equation for the sum of F = ma.
     
  7. Dec 26, 2009 #6
    LOL .. now I'm fine with Cheeta .. but your explanation still doesn't make sense.
    Why?
    the question is asking for the acceleration toward the edge of a cliff, and the only person moving is Tarzan, and not Jane. Therefore, the question asks for the acceleration of Tarzan, and so there is no such thing as coupled system as what you've said. Jane's weight has nothing to do with Tarzan's acceleration too b/c again, the question is asking Tarzan acceleration as he reaches Jane.
    Note: the cliff edge is behind the river, and Jane is on the river. Look at the attached file. Remember what the question says "The vine runs horizontally to the edge of a cliff, then vertically where Jane (m=50kg) is hanging on,above a river filled with crocodiles"

    ok.
     

    Attached Files:

  8. Dec 26, 2009 #7

    berkeman

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    Staff: Mentor

    Of course Jane's weight is the driving force in the acceleration of both J & T. The vine pulls T horizontally because J's weight is pulling down on the vine. Not sure where the disconnect is here.... Unless I'm misreading it, this is a pretty standard question in intro physics.
     
  9. Dec 26, 2009 #8

    diazona

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    Homework Helper

    Shouldn't the overall diagram look like this?
    tarzancliff.png
     
  10. Dec 26, 2009 #9
    First, the vine does not act as a pully -> go back to what the question says. Jane is in a seperate vine, in a stable position.

    For Diazona, I do not think your diagram is correct b/c vine by definition is a weak-stemmed plant that derives its support from climbing. Look back at the question >> A sudden sleet storm has removed all friction. Friction stands for air resistence.

    It is important to read the question carefully before you answer it >> that goes for both of you.

    There is no solution for this problem at the back of the text, however if its diagram is as easy as Diazona's, then it can be solved easily.
     
  11. Dec 26, 2009 #10

    Doc Al

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    No where does it mention that Jane is on a second vine. Only one vine is mentioned.

    Diazona's diagram seems like a reasonable interpretation of this ambiguously presented problem. Why do you think friction stands for air resistance? Wouldn't you think that a sleet storm would coat the surface with ice, thus removing any friction that Tarzan could use to prevent his sliding towards the edge?

    Always good advice.

    That's what I would do.
     
  12. Dec 26, 2009 #11

    diazona

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    Homework Helper

    It goes for you too, you know... specifically concerning this part:
    (Generally speaking, it's also important not to dismiss out of hand the ideas offered by the people who are trying to help you. But I'm not complaining :wink:)
     
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