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Static/Kinetic Friction Problem

  1. Sep 18, 2008 #1
    1. The problem statement, all variables and given/known data
    "A block that weighs 50N is initially at rest on a rough horizontal surface. A 20N horizontal force is then applied to the block. If the coefficients of friction (static = 0.6, kinetic = 0.3), what is the magnitude of the frictional force on the block?"

    2. Relevant equations
    Frictional force = coefficient of kinetic friction x normal force
    Max static frictional force = coefficient of static friction x normal force

    3. The attempt at a solution
    Once again I'm worried this problem is misleadingly easy:

    Max static frictional force = coefficient of static friction x normal force
    Max static frictional force = 0.6 x (50N - 20N)
    Max static frictional force = 0.6 x 30N
    Max static frictional force = 18N

    Is this method correct?
     
  2. jcsd
  3. Sep 18, 2008 #2

    LowlyPion

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    No.

    You need to draw a force diagram on the block.

    What are the normal forces?

    What is the horizontal forces.

    If the block weighs 50N what is the maximum static frictional force it can resist?
     
  4. Sep 18, 2008 #3
    I think I just realized my mistake..

    The normal force of the block is the given 50N, and the horizontal force applied is 20N, so
    Max static frictional force = 0.6 x 50N
    Max static frictional force = 30N

    The max static frictional force exceeds the force applied horizontally (30N > 20N)
     
  5. Sep 18, 2008 #4

    LowlyPion

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    I think you are on to something.

    Good Luck.
     
  6. Sep 18, 2008 #5
    So if the max static frictional force is 30N, and you are only applying 20N of force to it, won't that make the magnitude of frictional force 20N (since the block won't move anyway)?
     
  7. Sep 18, 2008 #6

    LowlyPion

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    Yes indeed.
     
  8. Sep 18, 2008 #7
    Thanks I appreciate it!
     
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