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Homework Help: A Question about Frictional Forces

  1. Sep 21, 2009 #1
    1. The problem statement, all variables and given/known data
    A horizontal force of 15 N pushes a .50 kg book against a vertical wall. The book is initially at rest. If the coefficient of static friction= 0.6 and the coefficient of kinetic friction = 0.40, what is the acceleration of the book?

    2. Relevant equations

    static friction= coefficient of static friction x normal force
    kinetic friction= coefficient of kinetic friction x normal force

    3. The attempt at a solution

    First, I drew a free body diagram of a rectangle next to the wall. To the right, I had the force 15 N pointing towards the rectangle (the force exerted on the book). To the left, I had a force - 15 pointing towards the rectangle (the force exerted on the book by the wall). Pointing in the down direction, I had a vector mg. In the upward the direction the force is equal to the frictional force.

    Thus, the force in the x direction equals 0.

    Ff-MG= MAy
    Thus, Ay= (Ff-MG)/M

    Ff= coefficient of kinetic friction * normal force

    For some reason, I'm not getting a good answer. Can someone tell me where I am going wrong? Thanks!
  2. jcsd
  3. Sep 21, 2009 #2


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    Gold Member

    The maximum static friction force at the point of pending motion is (coef. friction)*N. The actual static friction force may be different. Does the block move?
  4. Sep 21, 2009 #3
    No it is not! I get it now! Thank you thank you thank you!
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