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How would you go about solving this Work-kinetic energy thorem?

  1. Nov 1, 2012 #1
    A 2.1x10^3kg car starts from rest at the top of a driveway that is sloped at an angle of 20 degrees with the horizontal. An average friction force of 4.0x10^3N impedes the car's motion so that the car's speed at the bottom of the driveway is 3.8m/s. What is the length of the driveway?
    What i did was:
    .5mv^2=gmD(cos20)
    .5(2.1x10^3)(3.8)^2=4.0x10^3d(cos 20) and got d= 7.1
    I check the back of y book and the answer is 5.1. What did I do wrong? Did I use the wrong formula? How do I go about solving this problem?
     
  2. jcsd
  3. Nov 1, 2012 #2

    rock.freak667

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    Homework Helper

    That frictional force is already along the slope, so the total energy mgh will be equal to the work done by the frictional force + ke at the bottom of the incline.
     
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