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Static Friction

  1. Oct 16, 2005 #1
    I really don't know where to find the acceleration of the object in this question.

    When you push a 1.8 kg book resting on a tabletop, it takes 2.25 N to start the book sliding. Hwat is the coefficient of static fraction?

    Any help would be greatly appreciated, I have already done the force diagram and equations, found Fg and Fn, I just don't know how you would find the acceleration of the object.

    Thank You!
  2. jcsd
  3. Oct 16, 2005 #2


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    Homework Helper

    [tex]F_\mu = \mu N[/tex]
    Fmu being the force caused by friction, mu the coefficent of friction and N the force the surface exerts on the object.

    Using Newton's III, in your problem, can you figure out the force exerted on the book by the table?
  4. Oct 16, 2005 #3
    I found out that it was 931N. But then where would I go with that? Because I would need to find the Ff then divide that by 931N.

    Thank You!
  5. Oct 16, 2005 #4


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    How did you get 931 N? That is a big force for a book with a mass of only 1,8kg:
    F = ma <=> a = F/m = 931 N / 1,8 kg = 517 m/s^2.

    If by Ff you mean Fmu, the force caused by friction, you already know its maximum magnitude.
  6. Oct 16, 2005 #5
    I was looking at the problem! I am sorry! I got 17.64 for the Force of book (Fn) on the table. Now, would why I take that since if I got that answer I would take Fn=17.64/mass=1.8 equals the acceleration. Wouldn't I need to find the Ff first then take that and divide it by the mass?
  7. Oct 18, 2005 #6
    Hey there!
    May I know whether you want to find the acceleration of the book when it begins to slide, or the coefficient of static friction in this question?
  8. Oct 18, 2005 #7

    Doc Al

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    Staff: Mentor

    Don't think that you need to know the acceleration in order to find the force! For one thing, Newton's 2nd law says that the net force equals ma. In this case, the net force is zero--it just barely begins to slide. So that won't help you.

    Note that you are given the friction force. Use the formula that Päällikkö gave for relating the maximum static friction to the normal force.
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