# Engine power and car acceleration

1. Nov 13, 2007

### linnus

1. The problem statement, all variables and given/known data
The Porsche® 911 GT3 has a 380 hp engine and a mass of 1.4×103 kg. The car can accelerate from 0 to 100 km/h in 4.3 seconds. What percentage of the power supplied by the engine goes into making the car move? Assume that the car's acceleration is constant and that there are 746 Watts/hp.

2. Relevant equations
p=w/t
k=1/2mv^2
a=v/t

3. The attempt at a solution

Not sure...

2. Nov 13, 2007

### catkin

Power of engine in Watts?

Energy to accelerate 1.4×10^3 kg from 0 to 100 km/h?

Power to do that in 4.3 seconds?

3. Nov 13, 2007

### Bill Foster

What will the kinetic energy of the car be at 100 km/h?

Power is change of energy per change in time. You calculate change in energy and you are given change in time.

4. Nov 13, 2007

### linnus

so you would do
power that it uses for it to reach its final velocity/total power right?

5. Nov 14, 2007

### catkin

Actually its a stupid question. Or maybe a trick question.

"What percentage of the power supplied by the engine goes into making the car move?" Trivially: we have no information about losses -- services such as airco, electrical power, servo etc and frictional losses -- so must assume these are zero. A growed up question would not simply ignore those. More seriously we have no information about how much power the engine supplies, only about the max it can supply. Maybe, just maybe, it wasn't running at max power RPM with the throttle mashed into the carpet. Ignoring losses ALL the power the engine supplies goes into making it move. Where else would it go?

"An airplane crashes on the border of USA and Canada. Where do they bury the survivors?"

"Have you stopped beating your wife yet?"

6. Nov 14, 2007

### Bill Foster

You find the power that it took to reach that speed. Actually, you find the ENERGY it took to reach that speed, then divide it by the time it took to reach that speed to get power.

Then you divide that my the engine's power to get a fractional part.