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Homework Help: Finding Power

  1. Nov 14, 2005 #1
    What power must a man expend on a 100-kg log that he is dragging down a hillside at a speed of 0.50 m/s ? The hillside makes an angle of 20 degrees with the horizontal and the coefficient of friction is 0.9

    m = 100kg
    v = 0.50 m/s
    [tex] \theta [/tex] = 20 degrees
    [tex]\mu[/tex] = 0.9

    From here I thought that [tex] W_{man} + W_{f} = \Delta U[/tex]

    So, [tex] W_{f} = - \mu mgL \cos{\theta}[/tex] (that I understood), from here the books states that [tex]\Delta U = 0 - mgL \sin{\theta}[/tex] Where did the book get this? What does the 0 represent? How do I find the Power?

    The book then says that [tex] P_{man} = \mu mgv \cos{\theta} - mgv \sin{\theta} = 247J [/tex]

    Can someone walk me thru this?
  2. jcsd
  3. Nov 14, 2005 #2

    Chi Meson

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    U is the proper variable symbol for potential energy, gravitational potential energy in this case.

    The [tex] mgL \sin{\theta}[/tex] is another way of say "mgh," but on this slope (think triangle), h is the opposide side where L is the hypotenuse. mgh is the change in potential energy. The work done by the guy has to do two things: change the potential energy of the log, and overcome friction.

    Conceptual rant:
    Depending of conventions and definitions, sometimes the work done to overcome friction is not called "work" but "the mechanical equivalent of heat." That last bit is a mouthful, and I haven't seen it too often in textbooks lately, so I guess it's OK to call it work; the only problem is that it is not a transfer of mechanical energy if it turns to thermal energy, it should be called heat. Alas this distinction is low on the scale of hair-splitting importance.[/concpetual rant]
  4. Nov 14, 2005 #3
    What instrument do you play hemidemisimiquavers? I thought 16th notes were fast enough... geez, I can only imagine 64ths
  5. Nov 14, 2005 #4

    Chi Meson

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    I can handle short trills on the piano that are "hemis.":approve: As long as its with fingers 2 & 3 (the thumb is #1 on the piano)
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