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Homework Help: Tricky Energy Problem

  1. Nov 18, 2008 #1
    1. The problem statement, all variables and given/known data
    There is a car of mass 1900kg that travels at a steady speed of 27 m/s up and down a hill. The force of friction is the same in both directions and it takes 49 more horsepower to go up the hill than down the hill. What is the inclination of the hill.

    Given: mass of car= 1900kg , velocity = 27 m/s, difference in power =49 hp =35000 W.

    I set it up as an energy problem. First I got rid of time and set that for every second the car traveled 27 m and the difference in the (change in) energy was 35000 J.

    2. Relevant equations

    3. The attempt at a solution
    Step 1: I set ∆PE(up) -∆PE(down) = 35000J
    Then 2∆PE(up)=35000J

    Step 2: I used potential energy

    h= 35000J/[2(1900kg)(10 m/s2) = .92 m

    Step 3: A set up a right triangle: hypotenuse=27m and height=.92m

    I used theta=arcsine (.92m/27m) = 1.95 Degrees.

    I don’t know the answer but I think that I did everything correctly. Can anyone find a mistake?
  2. jcsd
  3. Nov 19, 2008 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Looks right to me!

    You might want to give the final answer to 2 sig figs though.

    Good job.
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