# Car Engine Energy Problem

Here I have a hard one... A car with the mass 1100kg accelerates from 0km/h to 50km/h in 9,0s. In this time intervall the engine exhausts an power of 57kW. How many procent of the power the engine exhauts is used to increase the moveable energy...

For this I take :

Wres = Fres = Wk

Wk= 1/2 M * v^2

First I take Km/h and transform it to m/s 50km/h = 13,8 m/s

then

550kg *13,8^2 = 104742 J Totalty Cars Moveable power

Then to get Fres I use

S=1/2(v-v0)t

S=13,8/2 * 9 = 62,1m

To get Fres I use Wres = F*S

62,1 * 104742 = 6504478,2J = 6504kJ

57kJ/6504kJ=0,0008 = 0,8 procent

Which Is TOTALY WRONG plz help me what shall I do!!!!!??? It shall be around 21%

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Doc Al
Mentor
You started out OK, but then went off track. How much power actually goes to increasing the speed of the car? Hint: What's the increase in KE? How much time does that take?

The KE is 104742J = 104kJ then shall I take 57/104 which is... 0,54 which is 54% which is also totally wrong ...

Doc Al
Mentor
Of course it's wrong. You need to compare power (total) with power (used to accelerate car), not with the KE of the car.

You found the KE; now find the power. (That's why I asked you "How much time does that take?")

Thx man forgot that effect is P = W/T!!! You should have sade Effect and it should have popped up directly ... well anyway you helped me thx man
So The answer is 104742/9 = 11638W = 11,6 kW

11,6kW/57 = 0,209= 0,21 :)