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Homework Help: Calculating kinetic friction

  1. Apr 22, 2009 #1
    A 23.0 kg block is initially at rest on a horizontal surface. A horizontal force of 76.0 N is required to set the block in motion. After it is in motion, a horizontal force of 60.0 N is required to keep the block moving with constant speed. Find the coefficients kinetic friction from this information.

    my attempt:
    After the block is in motion 60N is required to prevent it from accelerating.
    Therefore what is left over must be used to overcome friction which is 16N.
    Friction force = weight x coefficient of kinetic friction
    16N=23x9.8 x ck
    But this is incorrect.
    What am i doing wrong?
  2. jcsd
  3. Apr 22, 2009 #2


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    Science Advisor
    Gold Member

    60N is the force required to prevent it from accelerating, this is the same force to overcome friction. 16N isn't anything in this problem, it's just the difference between the force required to overcome static friction and the force required to overcome kinetic friction.

    Replace 16N with 60N in your calculations and try again.
  4. Apr 22, 2009 #3
    When friction force and the accelerative force are equal speed is constant. Anything extra will cause the body to accelerate so the accelerative force isn't overcoming friction.
    60n is used to determine the static friction.
  5. Apr 22, 2009 #4
    as you say you need the same force as friction to stay at constant velocity.

    The question tells you there there is 60 N of friction when moving.

    76 N is used to determine static friction.
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