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Homework Help: Power - horse pulling a sled

  1. Mar 7, 2010 #1
    1. The problem statement, all variables and given/known data

    A sleigh and driver with a total mass of 130 kg are pulled up a hill with a 15 degree incline by a horse

    If the overall retarding frictional force is 800 N and the sled moves up the hill with a constant velocity of 5.0 km/h,
    what is the power output of the horse? (Express in horsepower, of course.)

    2. Relevant equations


    3. The attempt at a solution

    I first converted the 5 km/h to m/s

    5km/h = 1.389 m/s

    Not sure what to do now. I know there is a retarding force of 800N and i know the velocity so i want to say i can find the force by the horse to travel at that velocity with that retarding force but im not sure how. cause then i believe if that force is found i could multiply that by the velocity to get the power in watts then convert to horsepower.

    thanks for any help :)
  2. jcsd
  3. Mar 7, 2010 #2


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    Homework Helper

    it is a "block on a ramp, with friction" ; sum the force components parallel to the ramp.

    or , see how much PE changes each second,
    and how much Work is done against friction each second .
  4. Mar 7, 2010 #3
    I think i am supposed to solve it using PE and Work because that is what my professor just went over.

    The thing is i feel like i dont have enough information to find the Power.

    I know potential energy is = m(g)(h)

    Im just not sure how to apply that to this problem.

    and to find the Work done against friction if the distance is not known how do i find this?

    Would i use

    W = change in kinetic energy

    W = 1/2mv_f^2 - 1/2mv_i^2

    v_i = 0 so

    W = 1/2mv_f^2

    Thanks for the help :)
  5. Mar 7, 2010 #4
    I understand it using the component forces parallel to the incline.

    I used: Ff + Fgx - Fh = 0

    800 + 130(9.8)sin(15) = Fh

    Fh = 1129.7

    P = 1129.7(1.389)

    P = 1569.2 watts or 2.1 horsepower.

    But I would also really like to know how to use PE and W to find it though :) any tips?
  6. Mar 7, 2010 #5
    Could i get some help with this next part. I thought i did it right but its wrong :(

    Suppose that in a spurt of energy, the horse accelerates the sled uniformly from 5.0 km/h to 19 km/h in 6.0 s. What is the horse’s maximum instantaneous power output? Assume the same force of friction.

    I found the acceleration

    a = 5.278 - 1.389 / 6
    a = 0.648

    then i found the force of the horse (Fh)

    Ff + Fgx - Fh = ma

    800 + 130(9.8)sin15 - 130(0.648) = Fh

    Fh = 1045.495

    then i found the power

    P = Fh * V_avg

    P = 1045.495 ( 5.278 - 1.389 / 2)

    P = 2032.96 W

    P = 2.1 hp

    that was wrong so i thought maybe since it says the max power output i thought to use just the velocity max so....

    P = 1045.495(5.278) = 5518 W

    P = 7.4 hp

    but that was wrong also :(

    Any suggestions? Thank you
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