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Homework Help: A crate is pulled by a force (parallel to the incline) up a rough incline. The crate

  1. Oct 19, 2011 #1
    A crate is pulled by a force (parallel to the
    incline) up a rough incline. The crate has an
    initial speed shown in the figure below. The
    crate is pulled a distance of 9.17 m on the
    incline by a 150 N force.
    The acceleration of gravity is 9.8 m/s2 .
    a) What is the change in kinetic energy of
    the crate?
    Answer in units of J

    b) What is the speed of the crate after it is
    pulled the 9.17 m?
    Answer in units of m/s

    For part A i tried
    F-mg[sin theta - (friction x cos theta)] and got 135.232 but got wrong answer then i tried multiplying it by the distance and got it wrong.
    Also i tried using F - [ μ m g cos θ ] and got it wrong
    Someone please help me
     
  2. jcsd
  3. Oct 19, 2011 #2

    Delphi51

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    Homework Helper

    Re: A crate is pulled by a force (parallel to the incline) up a rough incline. The cr

    The question appears to be incomplete. Do you know the angle, initial speed and coefficient of friction? If so, your calculation of
    "F-mg[sin theta - (friction x cos theta)]"
    looks like a good start. Here F = 150, friction = μmg. The result of the calc is the total force on the mass, which you can use in F = ma to find the acceleration. Once you have that, you can use accelerated motion formulas to find the Vf and then the Ek to complete part (a).
     
  4. Oct 19, 2011 #3
    Re: A crate is pulled by a force (parallel to the incline) up a rough incline. The cr

    ok i got part a which was

    533.0427

    how do i get part B
    i tried

    V= square root 2(553.0427)/11
    but got the wrong answer
     
  5. Oct 19, 2011 #4

    Delphi51

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    Homework Helper

    Re: A crate is pulled by a force (parallel to the incline) up a rough incline. The cr

    Didn't you find the Vf in part (a) ? Once you found the acceleration, you would have used d = Vi*t + .5*a*t² to find the time and then
    Vf = Vi + a*t to get Vf. Only after knowing Vf are you in a position to do the energy calc for the (a) answer.
     
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