1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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

    3. The attempt at a solution


    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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Have you been taught..

    Work (aka Energy) = force * distance
    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!!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted