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Homework Help: Elevated Pulley

  1. Jun 29, 2010 #1
    1. The problem statement, all variables and given/known data
    Two objects with masses m1 of 10kg and m2 of 2kg are connected with a string over an elevated pulley as shown and start to slide in the presence of friction.


    If h=10cm and [tex]\mu[/tex]=0, where is m1 when the system comes to rest

    2. Relevant equations

    3. The attempt at a solution
    I have absolutely no idea with this one...
  2. jcsd
  3. Jun 29, 2010 #2


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    Do you mean in the presence of friction μ = 0?
  4. Jun 29, 2010 #3


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    This problem requires some interpretation. What does μ = 0 mean? Can it mean no friction? Yet the problem says "in the presence of friction." It might mean "don't worry about friction as a force acting on the mass when the mass comes to rest", but I am not sure. Have you posted the exact wording of the problem as given to you?
  5. Jun 29, 2010 #4
    Yep, that's one of the things that confused me. I assumed it meant ignore friction..
  6. Jun 29, 2010 #5


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    The question "where is m1 when the system comes to rest" is also a bit unclear. Suppose you could find angle θ. How would that help you determine where the mass is from, say, the edge of the table? The best you can do is to assume that the pulley has zero radius and find the distance of the mass to the center of the pulley.

    My guess for approaching this (not so well-designed) problem is to say that the equilibrium position is where the normal force exerted by the table on m1 becomes zero. You can find θ from this condition.
  7. Jun 29, 2010 #6
    It is stange, but we have no choice - we have to respect facts :smile:. There are three forces action on the m1: gravity, table reaction and string tension.

    IF there is no friction there is unbalanced horizontal component of string tension. It is non-zero force until theta angle lower than 90 deg. And I can't see anything to balance this force.

    This mean that system moves until point of force application locate preciselly under pulley!?!

    Like blue one on the picture below


    But it is confusion me too.
  8. Jun 29, 2010 #7
    Thanks anyway :) Any answer's better than none! Hopefully they don't put anything weird like that in my exam!
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