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Calculating the power of a car engine

  1. Jun 22, 2012 #1
    1. The problem statement, all variables and given/known data

    The driving power of an engine is, say 30 kN. the car drives for 5 seconds with an acceleration of 10 m/s2. The initial velocity is 0 m/s.
    1) What does the minimum engine power need to be for this?
    2) What is the power of the engine at the end of the acceleration?

    2. Relevant equations

    A=F*s
    v=v0+at
    s=v0t+at2/2
    N=A/t=F*v

    3. The attempt at a solution

    The first part confuses me the most. You can approach this 2 different ways:
    a) you find that s=125 m; then according to the formula N=F*s/t, you get that N=0,75 MW. That would be the minimum power needed to get through 125 meters with 5 seconds. But the way I see it, this would be the power if we are not dealing with acceleration.
    b) You find that the final velocity of the car is v=50 m/s. According to the formula N=F*v, you get that N=1,5 MW. But isnt this also the minimum power needed for this cenario? At the end of the acceleration the car would need to have this power. So that would also be the minimum engine power so the car could do what is described in the task.

    What am I missing? Which one ise correct?

    Also, is the second half of the task solved by b) as well?

    Thanks in advance,
    fawk3s
     
  2. jcsd
  3. Jun 22, 2012 #2

    BruceW

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    Homework Helper

    The question is badly worded, and confusing. Firstly, they say "The driving power of an engine is, say 30 kN" So they say power, but give a force. I think you have correctly interpreted it to mean the force the engine produces. And I think you have done part 2 correctly. I can't think of what they want for part 1... You have worked out the time average of the power. (Since force is constant, force times distance is the total work, i.e. the power integrated over time, then you have divided by total time, so you end up with the time averaged power). This doesn't really sound like 'minimum engine power', but then I can't think of any other answer for part 1 which might give them the answer they want... whatever that is..
     
  4. Jun 25, 2012 #3
    From what I can understand, the "minimum" means the power the engine must be capable of achieving in order for this scenario to work. I actually think this should be the same as the part 2 has it, because if the force which the engine produces is constantly 30 kN, and at the end of the scenario the speed of the car is 50 m/s, then the power ought to be 1,5 MW. As the car needs to achieve this power in the end, this ought to be the minimum power the car MUST be capable of achieving.

    Am I not right?
     
  5. Jun 28, 2012 #4

    BruceW

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    Homework Helper

    yes, I would agree. But it seems weird that the two questions ask for the same thing.
     
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