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Power needed to keep car moving

  1. Nov 22, 2007 #1
    1. The problem statement, all variables and given/known data
    A 1200 kg car slows down from 90km/h to 70 km/h in about 5.0s on the level when it is in neutral. Approximately what power ( in watt's and hp ) is needed to keep the car travelling at a constant 80km/h?

    2. Relevant equations
    P = W / t

    3. The attempt at a solution
    I really have no idea ............ and
    how much does 1hp = in watts and in joules ?
    550 ft lbs/s = in joules and watts?
  2. jcsd
  3. Nov 22, 2007 #2


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    if you have the change in velocity and the time it acts for....you can find acceleration and F=ma...so you can find the power
  4. Nov 22, 2007 #3
    what does each of ur variables mean?? cause i think i use a different variables than u do and i find it confusing to understand
  5. Nov 22, 2007 #4


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    Oh well from the definition of power, the rate at which work,W, is done =[itex]\frac{dW}{dt}[/itex] since work done= Force(F)*distance(s) i.e.[itex]\frac{dW}{dt}=\frac{d}{dt}(Fs)[/itex] since force is constant you can then re-write it like [itex]\frac{d}{dt}(Fs)=F\frac{ds}{dt}[/itex] and well [itex]v=\frac{ds}{dt}[/itex]

    In short: F=Force of car
    v=velocity of car
  6. Nov 22, 2007 #5
    wha ??? so the answer is????
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