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Homework Help: Calculating Work With Friction

  1. Dec 18, 2008 #1
    Hey guys,
    I got a test tommorow regarding Work, Efficiency, Conservation of Mass, and all those other things. And I needed help with this question regarding work with friction. Oooh, I wish I was good in physics :( If anyone knows the answer, please post and tell me how to do it properly. Many thanks!
    1. The problem statement, all variables and given/known data
    A toddler pushes a chair at a constant speed with a force of 25.0N (Applied Force = 25N to the right) for a distance of 2.5m, but the chair is being pushed across a smooth floor against a force of friction (resistance of 10.0N) (Force of friction = 10N to the left). How much work is being done?

    2. Relevant equations

    Force of gravity=mg

    Net force=ma

    3. The attempt at a solution
    Well, I figured that I would subtract 10N from 25N and then use the result (15) and calculate it by the displacement, which is 2.5m. But when I checked my answer at the back of my physics textbook, my answer was incorrect. Now, I'm pretty much stuck? Would the force of friction be of the same value as the applied force?
  2. jcsd
  3. Dec 18, 2008 #2


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    Homework Helper

    What they give you is the force against which you are doing work.

    It's 10N.

    That's what you use to calculate the Work.

    Now the chair is not being accelerated so evidently the magnitude of the force the toddler is supplying is greater than needed if it was all in the direction of motion. This apparently means then that the force is at an angle and the horizontal component of it is 10N.
  4. Dec 18, 2008 #3
    oh, I see.

    So, would the equation be as follows?

    Work= Applied Force * Displacement
    = 10N * 2.5m
    = 25J
  5. Dec 18, 2008 #4


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    Homework Helper

    That looks correct.
  6. Dec 18, 2008 #5
    Thanks! :)
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