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Homework Help: Friction Help

  1. Feb 13, 2010 #1
    Problem: Ive been assigned a problem that involves a block on a table that is attached by a string to a block hanging over the edge. They are of different masses, and I am given coefficients for both static and kinetic friction. The former is .50, and the latter is .30. I have to find the acceleration of the system if it is released from rest.

    My problem: ok, so do I need to use the coefficient of kinetic friction at all? I do not think so...that's pretty much my question.
  2. jcsd
  3. Feb 13, 2010 #2
    I got a negative answer..
  4. Feb 13, 2010 #3
    Can you show what equations you used? Kinetic friction should be used for surfaces moving relative to each other.
  5. Feb 13, 2010 #4


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    Hi ƒ(x)! :wink:

    Do good ol' Newton's second law on each block separately, plus the fact that their accelerations must be the same (because the string length is constant).

    What do you get? :smile:
  6. Feb 13, 2010 #5
    m1 (on table) = 10 kg
    m2 = 4 kg

    .5*Fn = Fs
    .5*10*9.8 = 49 N = Fs

    Fnet = Fx - Fs = 4*9.8 - 49 = -9.8 N

    ma = -9.8
    a = -9.8/m = -9.8/(4+10) = -.7 m/s/s
  7. Feb 13, 2010 #6


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    ohh! you didn't give the masses before :rolleyes:

    no wonder you got a negative answer
    That doesn't make sense … how can the mass be accelerating upward?

    What does it mean if the weight is less than the µsN ?
  8. Feb 14, 2010 #7
    I'm guessing it means that the system isn't moving.
  9. Feb 14, 2010 #8


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    (why guessing? :rolleyes:)

    That's right! …

    if the system is released from rest, it will never move (even though if it was given a little nudge, the low µk would enable it to keep accelerating). :smile:
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