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Homework question. Please Help

  1. Nov 22, 2006 #1
    Hi everyone. Got this question as part of a homework assignment and become really stuck.Im usually ok on stuff like this but missed the lecture so cant get hold of any notes on rolling resistance. Please help

    A new sports car has the following specificationsand performance when fitted with 1 1.8 litre, 4 cylinder, 16 valve double overhead cam , turbocharged engine.
    Maximum Power: 190bhp at 7800rpm
    Max Speed : 140mph (level road, still air)
    Mass = 1200kg
    Chassis dimensions (approx): 1.7 x 1.4 x 4.2m (width x length x height)
    Coefficent of drag for the vehicle is 0.34
    Sea level air density is 1.2kg/m3

    A) If the rolling resistance of the vehicle at 140mph is 200N, estimate the efficency of the power transmission system in converting engine power to vehicle motion.
    B) Assuming the rolling resistance is directly proportional to vehicle speed and that the efficency of the power transmission system remains constant.Calculate the speed of the vehicle at 70%of max power.



    Thanks Everyone
     
    Last edited: Nov 22, 2006
  2. jcsd
  3. Nov 22, 2006 #2

    marcusl

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    Science Advisor
    Gold Member

    Air drag is 1/2*Cd*(air density)*speed^2*area
    so you need to estimate the car's frontal area.
    Power is F*dx/dt=F*speed
    Force F = drag + rolling resistance
    You should be able to figure it out from there.
     
  4. Nov 22, 2006 #3
    hey thanks. understand it now
    x
     
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