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Homework Help: Constant Power

  1. Sep 26, 2004 #1
    A car manufacturer claims that their car can accelerate from rest to 87 km/hr in 6.3 s. The car's mass is 940 kg. Assuming that this performance is achieved at constant power, determine the power developed by the car's engine.
    Ok, I solved the problem by first solving for constant acceleration. a=[v(f)-v(0)]/t
    Then using that I found the Force, using F=ma. Then plugging into P=F(dot)v

    The answer that i got, 87143.056W is wrong. I think it could be because the problem says it is constant power, not constant acceleration.

    How can I go about solving this problem? I dont really understand the concept of constant power instead of constant acceleration
  2. jcsd
  3. Sep 26, 2004 #2
    Remember what power is. The unit for power, watts, is equal to joules per second. Power is energy transfered in a given amount of time. Can you solve the problem using energy concepts?
  4. Sep 26, 2004 #3
    ok, i know that Kinetic energy is = .5mv^2

    so the kinetic energy at v=87 km/hr (24.167 m/s) is .5(940 kg)(24.167 m/s)^2= 11358.49 J. I dont know where to go from here.
    I divided it by 6.3 sec to get 1802.93 J/s, but that is wrong. Atleast the units are right, heh.
  5. Sep 26, 2004 #4


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    I think you have the right idea and your numbers look right. Was there any discussion of thermodynamic efficiency in your class or your textbook?
  6. Sep 26, 2004 #5
    in the last lecture the prof. talked about it for a few minutes at the end. my book has some examples, but nothing related to this problem, (or so i think)
  7. Sep 26, 2004 #6


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    That may be a big clue! Just to get an idea is "the answer" approximately 3 times as large as the one you got?
  8. Sep 26, 2004 #7
    well i won't know the right answer until the computer says "OK"after i enter it. until then it just says "NO"

    I'm curious, why would it be 3 times larger?
  9. Sep 27, 2004 #8


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    It's only a guess but thermodynamic efficiency is generally about 30%

    If I might ask, how do you know the answer you got is wrong?
  10. Sep 27, 2004 #9
    it says "no" when i put in a wrong answer
  11. Sep 27, 2004 #10


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    24.167^2*940/2 = 274500 :smile:

  12. Oct 4, 2004 #11

    How would you find this same type of problem, but in addition, there's a constant air resistance force? Thanks.
  13. Oct 4, 2004 #12
    Subtract work done by frictional forces from the total energy. (W=FdcosΘ)
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