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Please help - Efficiency of car engine question Unit test tomorrow

  1. May 8, 2013 #1
    Please help -- Efficiency of car engine question... Unit test tomorrow

    1. The problem statement, all variables and given/known data
    A car's engine is only 12% efficient at converting chemical energy in gasoline into mechanical energy. If it takes 18000 N of force to keep the car moving at a constant speed of 21 m/s, how much chemical energy would be needed to move the car a distance of 450 m at this speed?


    2. Relevant equations
    (Eout/Ein)100%
    Ek=1/2mv^2
    Et=Ek+Eg


    3. The attempt at a solution

    (Eout/Ein)100%

    18000N x 450m/ 18000N x 21 x Ek = 12%
    and then I solved for energy



    I know this is completely wrong but i just don't understand how to do this question without having the mass... I have my unit test tomorrow and this is the only question I don't understand on the review!! PLEASE help me i'm having trouble understanding the efficiency questions, especially this one!!! I just don't know how else to attempt it!!
     
  2. jcsd
  3. May 8, 2013 #2

    CWatters

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    Science Advisor
    Homework Helper

    Have you been taught..

    Work (aka Energy) = force * distance
    or
    Power = force * Velocity

    EDIT: Sorry I keep getting interrupted here.

    The definition of efficiency is...

    Efficiency(%) = 100 * Energy out / Energy in

    Rearrange to give Energy In..

    Energy In = 100 * Energy Out/Efficiency

    substitute force * distance for Energy Out

    Energy In = 100 * force * distance/ Efficiency
     
    Last edited: May 8, 2013
  4. May 8, 2013 #3
    thanks you!!
     
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