# Drag race- avg power output

1. Sep 26, 2009

1. The problem statement, all variables and given/known data
A top fuel drag racer with a mass of 500.0 kg completes a quarter-mile (402 m) drag race in a time of 4.3 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.

2. Relevant equations
P = W/T

3. The attempt at a solution
P = W/T
W=FD
F=MA =500*26.744 = 13372
..plugged in for W/T ... (13372 * 402 /4.3 )/ 1000
This has not gotten me far!

2. Sep 26, 2009

### rock.freak667

Using a kinematic equation in the first 4.3 seconds it travels 402m starting from rest, so what is its acceleration? When you get this, what is the force exerted by the car then?

3. Sep 26, 2009

### Delphi51

Yes, how did you get that 26.744 m/s^2? I get two different answers for the acceleration depending on whether I use the distance of 402 m in 4.3 s or the final velocity of 115 m/s in 4.3 s. I think that means the acceleration is not constant, and so you can't do the problem using the W = FD method (because the force isn't constant either).

I suggest you start with the P = W/t but instead of W = Fd, use the final energy minus the initial energy for the W.

4. Sep 26, 2009

I got acceleration by vf-v0/2.
However I got the answer. I did P=FV for avg velocity.
Thanks though!

5. Sep 26, 2009

### RoyalCat

I think a better approach here would be to use the work-energy theorem:

$$W=\Delta E_k$$

6. Sep 26, 2009

RoyalCat,
I cannot see the formula u posted. please try it again

7. Sep 26, 2009

### Delphi51

That doesn't make sense; it gives an average velocity but not an acceleration. Wrong units for one thing.

RoyalCat's formula says W = final energy - initial energy
and is the same thing I suggested earlier.

8. Sep 26, 2009