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Homework Help: Power formula question

  1. Oct 31, 2005 #1
    A 1000Kg auto travels up a 3 percent grade at 20m/s. Find the horsepower required, neglection friction.
    ok HP, that means power formula
    work/time right, or f * v
    how do i approach here
    i wanted to do v2 squre = v1 square + 2 ad. but i dotn know if v1 is 0 and is distance (3/100)?
    i did that and i got a wierd acceleration number, plugged that into f = ma, got force and mulitplied that by V which is 20 and i got the wrong answer, any ideas?
  2. jcsd
  3. Oct 31, 2005 #2

    Chi Meson

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    Wrong direction. There is no acceleration here. The 20 m/s is implied to be constant (it never says "starts at rest" or the like, only "travels at 20 m/s).

    power is F*v as you noticed. In this situation, the force used to maintain constant speed is equal to the resistave forces. Since there is no friction, the only resistive force is the component of weight that points downhill, parallel to the slope.

    Or (to do the same thing differently) you could take the full weight at the resistive force and multiply this by the vertical componant of the speed (how much of the 20 m/s points straight up vertically?)

    Have you done trigonometry using "gradients" instead of degrees?
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