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Homework Help: Energy Problem

  1. Oct 25, 2005 #1
    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


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

    Then to get Fres I use


    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%
  2. jcsd
  3. Oct 25, 2005 #2

    Doc Al

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    Staff: 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?
  4. Oct 25, 2005 #3
    The KE is 104742J = 104kJ then shall I take 57/104 which is... 0,54 which is 54% which is also totally wrong ...
  5. Oct 25, 2005 #4

    Doc Al

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    Staff: 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?")
  6. Oct 25, 2005 #5
    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 :)
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