1. May 8, 2013

### 13physicsdude

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. May 8, 2013

### CWatters

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
3. May 8, 2013

thanks you!!