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Homework Help: Friction has 2 directions?

  1. Oct 18, 2012 #1
    1. The problem statement, all variables and given/known data
    http://www.xtremepapers.com/papers/...S Level/Mathematics (9709)/9709_w11_qp_43.pdf
    number 6, find 2 tension


    2. Relevant equations
    F=μR


    3. The attempt at a solution
    Okay, so R(reaction force)=Tcos30 Tsin30=F+20
    the first tension is found, 68.5.

    However, Im not understanding how there can be two tensions, as a result of frictions of 2 directions?? the marking scheme says the other friction is to prevent downward motion, but the tension isnt even downwards? How can there be 2 Tensions at the same time
     
    Last edited: Oct 18, 2012
  2. jcsd
  3. Oct 18, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The friction can act upwards or downwards, depending on how hard the string pulls.

    If the ring is on the verge of sliding up, the friction acts down; on the verge of sliding down, the friction acts up. Friction always opposes slipping between surfaces.
     
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