Please help - Efficiency of car engine question Unit test tomorrow

AI Thread Summary
The discussion revolves around calculating the chemical energy needed for a car's engine, which is only 12% efficient. To find the required energy, the formula for efficiency is rearranged to Energy In = 100 * Energy Out / Efficiency. The Energy Out is determined by multiplying the force (18000 N) by the distance (450 m). Substituting these values into the equation provides the necessary chemical energy input. The user expresses confusion about the problem, particularly regarding the efficiency concept, but the solution method is clarified through the efficiency formula.
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Please help -- Efficiency of car engine question... Unit test tomorrow

Homework Statement


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?

Homework Equations


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

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!
 
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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:
thanks you!
 

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