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Work and Kinetic Energy of weight

  1. Jul 24, 2009 #1
    1. The problem statement, all variables and given/known data

    A 20.0-N weight slides down a rough inclined plane which makes an angle of 30.0o with the horizontal. The weight starts from rest and gains a speed of 15.0 m/s after sliding 150 m. How much work is done against friction? Use g = 10 m/s2.

    2. Relevant equations

    W = Fd cos θ

    3. The attempt at a solution

    I substituted it in the equation and got around 2600 J but the answer is wrong.

    Can anyone help me out?

    Thanks a lot before hand
  2. jcsd
  3. Jul 24, 2009 #2


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    Homework Helper

    Try using conservation of energy.
    150m is the hypotenuse, what is the length of the opposite side of the triangle formed?
  4. Jul 24, 2009 #3

    That's because (for one) you used the wrong force.
    a). What force is burning up your mechanical energy: gravity or friction?
    b). Stated another way, Which force is non-conservative: gravity or friction?

    (Here's a hint: "Work done against friction" = minus "the work done BY friction")
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