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Constant Power, Variable Acceleration

  1. Jun 29, 2011 #1


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    1. The problem statement, all variables and given/known data

    A car of mass m accelerates from rest along a level straight track, not at constant acceleration, but with constant engine power, P. Assume that air resistance is negligible.

    a) Find the car's velocity as a function of time
    b) A second car starts from rest alongside the first car on the same track, but maintains a constant acceleration. Which car takes the initial lead? Does the other car overtake it? If yes, write a formula for the distance from the starting point at which this happens.
    c) You are in a drag race, on a straight level track, with an opponent whose car maintains a constant acceleration of 12.0 m/s2. Both cars have identical masses of 1000 kg. The cars start together from rest. Air resistance is assumed to be negligible. Calculate the minimum power your engine needs for you to win the race, assuming the power output is constant and the distance to the finish line is 0.250 miles.

    2. Relevant equations

    P=dW/dt; W=F [dot] delta-r; F=ma

    3. The attempt at a solution

    I'm pretty stuck at this one. My first thought is dW/dt=F [dot] dr/dt = F [dot] v = Fvcos[F-v]=P, but since a (and therefore F) are variable, I'm not sure where to go from here. Once I get the answer to A, I think I can do B and C (though the acceleration isn't mentioned in B, which I find confusing). Any hints? Thanks :)
  2. jcsd
  3. Jun 29, 2011 #2


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    Staff: Mentor

    Constant power means that energy is being given to the car at a constant rate. Where is that energy going?
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