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Work, potential and kinetic energy help

  1. Dec 16, 2012 #1
    1. The problem statement, all variables and given/known data
    A 30 kg block is slid down an inclined plane 5 m long with a height of 3 m from the floor. If the force of friction is 50 N, find the work done by the friction and the final velocity at the end of the plane.


    2. Relevant equations
    W = F*d
    mgh = (1/2)mv^2

    3. The attempt at a solution
    50 N * 5 m * cos(180) = -250 J

    Incorrect

    30 kg * 9.8 m/s^2 * 3 m = (1/2) * 30 kg * v^2
    882 = 15*v^2
    58.8 = v^2
    v = 7.6681 m/s

    Incorrect

    I don't know how else to approach this.
     
  2. jcsd
  3. Dec 16, 2012 #2

    Doc Al

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    Staff: Mentor

    Nothing wrong with this. They probably wanted just the magnitude of that work.

    You forgot about the friction.
     
  4. Dec 16, 2012 #3
    The answer says -150 J so I guess that's an error then.

    How do I account for fricton in potential energy?
     
  5. Dec 16, 2012 #4

    Doc Al

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    Staff: Mentor

    Looks like they used the 3 m distance by mistake.
    Initial mechanical energy + work done by friction = Final mechanical energy
     
  6. Dec 16, 2012 #5
    So then I get (30 kg * 9.8 m/s^2 * 3 m) + (-250 J) = (1/2) * 30 kg * v^2

    I get 6.49 m/s which is also incorrect (solution is 7.3 m/s).
     
  7. Dec 16, 2012 #6

    Doc Al

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    Staff: Mentor

    I agree with your answer. Even using their incorrect value for the work done by friction you won't get their answer.

    What book is this from?
     
  8. Dec 16, 2012 #7
    Something our professor wrote :-p

    It's so intimidating because I feel like I'm not getting the concepts when it just turns out the solutions are incorrect...so I'm still not confident enough to say that my answer is correct and the answer key is not
     
  9. Dec 16, 2012 #8

    Doc Al

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    Staff: Mentor

    It is unfortunate when the professor makes errors, but it happens.

    Your textbook should have problems for you to work on. Often some of the answers are given.

    You could also supplement your text with a problem book, such as a Schaum's Outline.
     
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