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Problem about finding the work done by friction force

  1. Sep 16, 2011 #1
    1. The problem statement, all variables and given/known data

    A block of mass M=3.4 kg is dragged over a horizontal surface by a force F=20.0 N. The block is displaced a distance d=17.0 m, the friction force Ff is 3.3 N. What is the work done by the friction force?


    2. Relevant equations

    Ff=μkFn

    3. The attempt at a solution

    I solved a problem before this one in finding the applied force but this one is very confusing to me. I have no idea where to even start. Thank you!
     
  2. jcsd
  3. Sep 16, 2011 #2
    Work done by friction is always negative. The work done by a constant force is always the dot product of the force multiplied the displacement whether or not the force is the cause of the displacement. So the work done would be -3.3*17 = -56.1 Joules.
     
  4. Sep 16, 2011 #3
    Why don't we start with recalling how to calculate the work done by any Force?
    [itex] \Large W = \int \vec{F} \cdot d\vec{r} [/itex]
    We can reduce this to simply:
    [itex]
    \vec{F}\cdot{\vec{x}}
    [/itex]
    Now the rest is pretty straightforward in your case.
    Hope that takes care of the confusion,
    Daniel
     
  5. Sep 16, 2011 #4
    Thank you so much!!!
     
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