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Homework Help: Friction/forces problem

  1. Sep 14, 2011 #1
    1. The problem statement, all variables and given/known data
    The coefficient of static friction between a block and a horizontal floor is 0.44, while the coefficient of kinetic friction is 0.10. The mass of the block is 5.4 kg and it is initially at rest.

    (a) What is the minimum horizontal applied force required to make the block start to slide?
    (b) Once the block is sliding, if you keep pushing on it with the same minimum starting force as in part (a), does the block move with constant velocity or does it accelerate?
    (c) If it moves with constant velocity, what is its velocity (in m/s)? If it accelerates, what is its acceleration (in m/s2)?

    2. Relevant equations
    F = ma
    fs = usN

    3. The attempt at a solution

    a) F=ma
    F=(5.4kg)(9.8m/s/s) = 52.92, which isn't right

    b) I believe it will accelerate.

    c) It's not 9.8 m/s/s. I will probably have to find it via F=ma?

    Thanks so much for your help!
  2. jcsd
  3. Sep 14, 2011 #2
    Well, I figured out part a! a = 5.4 kg * 9.8m/s/s *.44 = 23.3. Still unsure about the rest though.
  4. Sep 14, 2011 #3


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    Gold Member

    You can answer the rest by using Newton's 2nd Law, Fnet = ma, where Fnet is the algebraic sum of the applied force and the friction force. Watch plus/minus signs.
  5. Sep 15, 2011 #4
    Thanks so much, got it now!
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