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Power as a car going uphill

  1. Nov 18, 2008 #1
    1. The problem statement, all variables and given/known data
    The mass of a car is 1500 kg. The shape of the body is such that its aerodynamic drag coefficient is D = 0.380 and frontal area is 2.50 m2. Assuming that the drag force is proportional to v^2 and ignoring other sources of friction, calculate the power required to maintain a speed of 100 km/h as the car climbs a long hill sloping at 3.50° the answer is in horsepower

    2. Relevant equations
    R = .5DpAv^2
    P = Fv
    1hp = 746W

    3. The attempt at a solution
    Total force the engine needs to overcome is the gravitational force at an angle and air resistance.
    To find force, F = mgsin(theta) + .5 DpAv^2. Plugs everything in, F = (1500)(9.8)sin(3.5) + .5(.38)(1.29)(2.5)(27.78^2) = 1370.3N
    To find power, P = 1370.3(27.78) = 38067W, 38067W = 51 hp.

    I don't think my answer is correct. Can anyone offer any insight as to why? Thank you in advance.
  2. jcsd
  3. Nov 18, 2008 #2


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    Looks correct to me. You probably were expecting a bit more horse required?
  4. Nov 19, 2008 #3
    I entered my answer into the online homework thing and it told me I was 10% within the correct answer. I just can't figure out what's wrong with what I did. Man...
  5. Nov 19, 2008 #4


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    Did you copy down the correct information given in the problem?
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