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Homework Help: Frictional Force vs normal force

  1. Aug 5, 2015 #1
    1. The problem statement, all variables and given/known data
    A 10 N horizontal force is applied to a 50 N block initially at rest on a rough horizontal surface. If the coefficient of static friction is 0.5 and the coefficient of kinetic friction is 0.4, then the frictional force on the block is
    • A

      10 N

    • B

      20 N

    • C
    • 0 N

    • D

    • 25 N
    2. Relevant equations
    Frictional force = (coefficent of static friction)(normal force)
    Normal Force = mass x gravity = 50 N due to newtons second law

    3. The attempt at a solution
    Frictional force = 50 * 0.4 = 20 N but the answer is 10 N and I don't understand why
  2. jcsd
  3. Aug 5, 2015 #2
    With regard to static friction, the frictional force is not always equal to the coefficient of static friction times the normal force. If the frictional force does not exceed (coefficient of static friction)(normal force), then the object will not slide on the surface. In your case it doesn't, so no sliding will occur, and the body will be in static equilibrium, with the frictional force being in equilibrium with the 10 N applied force. In general, for static friction,

    (frictional force) ≤ (coefficient of friction)(normal force)

  4. Aug 5, 2015 #3
    okay thank you so much
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