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Grade 11 Physics Question?

  1. Jan 16, 2014 #1
    1. The problem statement, all variables and given/known data

    An object experiences a friction force, Ff, of 6.6 N and a normal force, Fn, of 30.0 N.

    a) What is the coefficient of friction, μ?
    b) Is the object moving? How do you know?

    2. Relevant equations

    μ = Ff / Fn

    3. The attempt at a solution

    I understand how to get the answer for a) which is 0.22:
    μ = Ff / Fn
    μ = 6.6 / 30.0

    As for b) though, I don't know if I'm overthinking it or something, but I think there aren't enough given variables to answer the question. The textbook points out that the question is K/U (in other words, it should be easy) but I can't figure it out.

    Thanks in advance.
     
  2. jcsd
  3. Jan 16, 2014 #2

    Simon Bridge

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    Welcome to PF;
    Can there be friction if something is not moving? Explain.
     
  4. Jan 17, 2014 #3
    There is not enough information to answer either part a or part b. If Ff/Fn does not exceed the coefficient of static friction, then the body is not moving, and you can't determine what the coefficient of static friction or the coefficient of kinetic friction is. All you know is that the coefficient of static friction is greater than 0.22.

    If the body is moving, then 0.22 is the coefficient of kinetic friction.

    Chet
     
  5. Jan 17, 2014 #4
    Well, I know that friction always opposes an applied force to an object, but if the applied force is not strong enough, the object won't move, and static friction hasn't been overcome.

    If the applied force is stronger than the static friction, the object starts to move, and the object experiences kinetic friction instead.

    So, to answer your question, there can be friction if something is not moving.
     
  6. Jan 17, 2014 #5
    Very nicely explained.
     
  7. Jan 17, 2014 #6

    lightgrav

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    so the friction coefficient is at least 0.22 ... that's a valid answer for a).
    Chet is wrong about b): if Ff/Fn < μ_static , the object might have been already moving.

    Carefully worded, Friction always opposes an impending slide along the surface.
     
  8. Jan 17, 2014 #7
    This is not always the case.
    Imagine you have a book laying flat on the table and a coin on top of the book.
    If you push the book it will slide on the table and the coin will move with the book (at least up to some maximum acceleration)
    What force will accelerate the coin? What is the direction of this force? What is the "applied force" in this case?
     
  9. Jan 17, 2014 #8
    I should have worded it a little better. If the body is not moving, Ff/Fn has to be less than or equal to the coefficient of static friction. If the body is moving, then Ff/Fn is equal to the coefficient of kinetic friction. Since the input information does not tell you whether the body is moving or not, you can't determine the answer for part a.
     
    Last edited: Jan 17, 2014
  10. Jan 17, 2014 #9
    Yes. This is correct. Depending on the situation, the friction force can act in either direction.
     
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