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A rope hanging over edge of table.

  1. Dec 18, 2009 #1
    The coefficient of statis friction between a rope and the table on which it rests is 0.3. Find the fraction of the rope that can hang over the edge of the table before it begins to slip.

    This is an extra credit problem so we haven't learned this yet. I don't know where to start but i'm guessing that to find that fraction of the rope, you need the mass,the weight and the gravitational force and stuff??
    cluless >.< please help.
     
  2. jcsd
  3. Dec 18, 2009 #2
    Rope hanging over edge of table.

    question
    The coefficient of static friction between a rope and the table on which it rests is 0.3. Find the fraction of the rope that can hang over the edge of the table before it begins to slip.

    Equation/ attempt at solution
    well, i googled this question and the equation i came up with was
    (0.3)(M-n[M/L])g=[n M/L]g .

    But as this is an extra credit problem and we haven't learned this yet, im not sure what i'm doing.
    Clueless, please please help.
     
  4. Dec 18, 2009 #3

    mgb_phys

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    Re: Rope hanging over edge of table.

    Draw a diagram of the forces.
    You have a weight pulling down on the hanging part, how will this weight depend on the length?
    This is balanced by the friction on the table part, friction = 0.3 * downward force (ie the weight of the part still on the table)

    If you think carefully the actual weight/metre of the rope doesn't matter, can you see why
     
  5. Dec 18, 2009 #4
    you don't need to now the length or mass of the rope to solve this. Only gravity and the coefficient of friction are needed.
    To solve you need to set the force of friction equal to the force of gravity.
    Keep in mind only the portion of the rope on the table will contribute to the force of friction and only the part off of the table will contribute to the force of gravity.
     
  6. Dec 19, 2009 #5

    Redbelly98

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    Moderator's note: two threads merged into one.

    Note to myaaxcore: please do not start multiple threads to ask the same question.
     
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