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Homework Help: Total Energy Calculation

  1. Mar 27, 2013 #1
    1. The problem statement, all variables and given/known data

    A car of mass 850 kg is driven at a steady speed of 70 km/hr up a hilly road
    with a slope of 30°. Using the macroscopic energy equation, determine the
    power delivered by the engine of the car.

    2. Relevant equations
    E= KE + PE
    E = (1/2)mC^(2) + mgz
    Power = E/t

    3. The attempt at a solution

    My attempt at a solution was:
    E = 1/2mC^(2) + mg(Ctsin(30))
    E = 1/2(850)(19.44)^(2) + (850)(9.81)(19.44)tsin(30)


    Power = (1/2(850)(19.44)^(2))/t + (850)(9.81)(19.44)sin(30)

    Why do I still have an unknown t? I am supposed to be able to solve it with the information given but I can't get it!
    Please please help!
    Last edited: Mar 27, 2013
  2. jcsd
  3. Mar 27, 2013 #2


    User Avatar

    Staff: Mentor

    Your problem is with
    You should be considering variations...
  4. Mar 27, 2013 #3
    Do you mean the variations in potential and kinetic energy as the car drives up the hill? I am not sure how to calculate that, would you be able to explain?

    I am sorry, I know it is such a simple question.
  5. Mar 27, 2013 #4


    User Avatar

    Staff: Mentor

    If you write ##P = E/t##, think what happens if the car is going at constant speed on a flat road.

    The power is used to change the energy of the car, so you have to consider ##P = \Delta E / \Delta t## or, even better, the instantaneous power ##P = dE / dt##.

    Hope this helps. Don't hesitate with further questions if it doesn't!
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