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Car going up incline

  1. Jul 11, 2014 #1
    1. The problem statement, all variables and given/known data
    Ok this question came up on a car forum and i tried to answer it, but I think my answer is way to simple and I solved the problem incorrectly
    1. A 2072lb Opel GT goes up 20 degree incline how much horsepower will the car need to get up the hill at the same speed as the same car on the ground. The car on the ground has 102 HP
    2. I didn't solve this one: Both cars now start from rest and travel a quarter mile. Assuming the car on the ground completes the distance in 18 seconds how much HP will be needed for the car on the incline to finish in the same amount of time. (accel is constant)
    Dont worry about friction or air resistance all things are equal.



    2. Relevant equations
    mgcosΘ and mgsinθ


    3. The attempt at a solution
    The force against a 9210.46 Newton (2072lb) Opel GT on an incline of 20° would be equal to mass*gravity(sin20°) which rounds to about 3153 newtons (708.8 lbs)
    So without any air resistance, friction, wheels falling off, fuel leaks, drivers, passengers, or dead bodies in rear console, your Opel would "weigh" 2780.8lbs.
    Let's say you have the top spec stock high comp GT with 102HP. Your HP to weight ratio would be .03668009 hehe (very roughly 1 hp per 27.8 lbs)
    On the ground your HP to weight ratio is 0.04922779! (Very roughly 1 HP per 20.7 lbs)
    Xhp/2780.8=.04922779
    Hp up hill would have to be about 137hp to equal the car on the level surface!
     
  2. jcsd
  3. Jul 11, 2014 #2

    haruspex

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    Doesn't seem to me to be enough information. I don't think you can simply take the ratio of the forces like that.
    Suppose the rolling resistance is F, the extra downslope force due to gravity is F', the speed is v, the power on the level is P, and the extra power up the slope is P'.
    Then you have
    Fv = P
    (F+F')v = P+P'
    You know P and F', but that leaves three unknowns and only two equations. You can deduce F'v = P', but that still doesn't help. In the attempt posted, you have effectively assumed F = weight of car, which is not true.
     
  4. Jul 12, 2014 #3
    Give your car some arbitrary drag co-efficients :
    Cd = air drag co-efficient (sea level) = 0.45
    Crr = wheel rolling resistance co-efficient = 0.03
    Level ground drive wheel power = 102 hp = 76,061 Watts
    Mass = 2,072 lbs = 934 kg
    Top speed of this vehicle = 115.45 mph
    Power required by this vehicle up a 20 degree incline at the same speed = 317.72 hp
     
  5. Jul 13, 2014 #4

    CWatters

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    As above.

    To work out the extra power needed going up hill you need to know or calculate the vertical speed. There isn't enough info in the original problem to do that. For example if the speed was slow (or the hill shallow) then the power required to overcome drag and rolling resistance would dominate and the same power would be required going up hill as on the flat.
     
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