# Average Power Output

1. Sep 19, 2008

### lpcampbe

A top fuel drag racer with a mass of 600.0 kg completes a quarter-mile (402 m) drag race in a time of 4.4 s starting from rest. The car's final speed is 115 m/s. What is the engine's average power output? Neglect friction and air resistance.

I know:
Power = Work/ change in time
W = Force * change in distance
F = mass * acceleration
Vf = Vi + at

so i solve for a
115 m/s = 0 m/s + a(4.4 s)
a = 26.1364 m/s^2

then i solve for F
F = 600 kg * 26.1364 m/s^2
F = 15681.8182 N

then i solve for W
W =15681.8182 N * 402 m
W = 6304090.909 J

then i solve for P
P = 6304090.909 J / 4.4 s
P = 1432747.934 W

I'm supposed to answer in kW
P = 1432.747 kW

I don't understand why this answer is wrong!

2. Sep 19, 2008

### LowlyPion

Perhaps you want to consider that

$$Work = \Delta KineticEnergy = \frac{m*v^2}{2} - 0$$

3. Sep 19, 2008

### lpcampbe

Thanks, LowlyPion!!

I also tried to find a using Vf^2 = Vi^2 + 2ax and i got this same answer, but the equation you gave me made my life so much easier. :)

Thanks again!!