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Homework Help: Calculate power with v on inclined plane with miu

  1. Jan 25, 2009 #1
    1. The problem statement, all variables and given/known data

    please advise how to solve. seems i can't without d given, or t!

    a man drags a 130 kg object down an inclined plane at a speed of 1.40m/s
    The coefficient of friction between the object and the inclined plane is 0.800 and the angle of inclination with the horizontal is 23 degrees. What is the amount of power this man expends in dragging the object?

    2. Relevant equations
    Work = F d
    Work = F cos theta d

    Power = Work/time
    Power = F v

    Work = change in kinetic energy

    3. The attempt at a solution

    F friction = 0.8 ( mg cos 23) = 95.7N
    Power = F.v = 95.7 * 1.4 =134W
    but i know this answer is wrong tho, the answer given is 629 W.

    please help me get this before my exam! many thx
  2. jcsd
  3. Jan 25, 2009 #2


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    Homework Helper

    Welcome to PF.

    I would suggest recalculating your frictional resistance. That looks a little light.
  4. Jan 25, 2009 #3
    Thanks for the heads up, i didn't multiply with gravity as i should have - the friction force corrected is 750.5N

    I think the parallel and net forces also play a role here in resisting friction. I can't seem to get the applied force down the inclined plane.

    the parallel force along the incline is mg sin 23 = 497.8N

    This should reduce the force acting down the incline ( F friction - f parallel) = 252.7 N
    but friction is still more and it should not move.

    how can i find the force needed to drag it down the incline at v= 1.4m/s? won't i need acceleration to find the net force?
  5. Jan 25, 2009 #4


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    Homework Helper

    There is no acceleration as it is at constant speed of 1.4 m/s.

    So yes you do subtract the Sin23*130*9.8 from the frictional maximum need to push it along, because as you have figured it is the net force that HE supplies that is doing the work that they ask for.
  6. Jan 25, 2009 #5


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    Homework Helper

    I would note however that :

    .8 * 130 * 9.8 * cos23 = 938.1785
  7. Jan 25, 2009 #6
    omg, i got it!! no net force, he just need to apply additional force of 449 N to overcome friction and move it at a constant speed. Multiply this force and velocity will give me the Power !!!! thx :D
  8. Jan 25, 2009 #7
    yup caught that too!! it was messing up my calculations!!! thx so much lowlypion! :D
  9. Jan 25, 2009 #8
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