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Homework Help: Car engine and constant power

  1. Apr 1, 2014 #1
    1. The problem statement, all variables and given/known data
    a cars engine produces constant power therefore the cars speed will increase at a constant acceleration.

    is this true or false

    2. Relevant equations
    so P=mad/t

    3. The attempt at a solution

    so if p is constant it appears a must be constant. Therefore the statement is true

    When I logically think about this situation I think it is false. I imagine myself driving down the highway in my stang turning 2500 rpm at 100 mph. Assume I stay in the same gear. Unless I raise the revs by giving it more gas I will not accelerate. When raising the engine rpm my engine makes more power....so how can I accelerate if my engine maintains a constant rpm(same power output)?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 1, 2014 #2
    Your equations are correct and so is your logic. However, can you think of any additional forces which may act against the acceleration of the car?
  4. Apr 1, 2014 #3
    Ah, wind resistance and friction between the road and tires...and then theres also the some power loss due to the weight of the drive train components.
  5. Apr 1, 2014 #4
    Yeah. I think it's funny to think about a bike. The wheels are so thin, but that little surface area combined with the air stops you from going as fast as you want.
  6. Apr 1, 2014 #5
    so just that little bit of resistance from the tires and the air hitting the body of the car prevents the car from accelerating,correct?
  7. Apr 1, 2014 #6
    so the answer would be true?
  8. Apr 1, 2014 #7
    Yes, so it's not that little really.
  9. Apr 1, 2014 #8
    Well in real life the answer is false for a car. A rocket in deep space could accelerate indefinitely.
  10. Apr 2, 2014 #9


    User Avatar

    Staff: Mentor

    Whoa! I'm not convinced that you have demonstrated it to be true. All I see you having shown here is that
    the factor (ad/t) is constant. I think you are asked to determine whether or not a is constant---am I correct in believing that is what the problem is asking?
  11. Apr 2, 2014 #10
    Yes that is what the problem is asking
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