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Homework Help: Power dissipation problem

  1. Oct 23, 2009 #1
    1. A certain car has an engine that provides a maximum power P_0. Suppose that the maximum speed of the car, v_0, is limited by a drag force proportional to the square of the speed (as in the previous part). The car engine is now modified, so that the new power P_1 is 10 percent greater than the original power (P_1=110\%P_0)

    Assume the following:

    * The top speed is limited by air drag.
    * The magnitude of the force of air drag at these speeds is proportional to the square of the speed.

    By what percentage, (v_1-v_0)/v_0, is the top speed of the car increased?

    2. F_drag = C_d*Av^2
    P = -C_d*Av^3

    3. P_1 is proportional to v_1^2
    Thus, 1.1P_o is proportional to (1 + alpha)^n * v_o^2

    Divide by P_o proportional to v_o^n to obtain:

    alpha = 1.1^(1/n) - 1

    My answer was 4.8% and was wrong. Again, I want the percentage increase (two sig figs) in speed: (v_1 - v_o)/v_o
  2. jcsd
  3. Oct 23, 2009 #2


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    Linus, I find the notation confusing. Could we just say F = kv^2 so
    P = F*v = k*v^3.
    You could then write another equation with 1.1P and some other letter for the faster velocity. It should be easy to eliminate the P between the two equations and come up with an equation relating the two v's. I get considerably less than 4.8% that way.
  4. Oct 23, 2009 #3
    I'm sorry but I don't understand how you're eliminating P, and furthermore how you are getting to a raw percentage.

    If I take P = kv_o^3
    1.1P = kv_1^3

    substitute for P in the 1.1P equation:

    1.1(kv_o^3) = kv_1^3
    1.1v_o^3 = v_1^3

    the new speed v_1 would be cuberoot(1.1v_o^3)

  5. Oct 23, 2009 #4


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    P = kv^3 and 1.1P = kV^3
    Put the first in the second:
    1.1*kv^3 = kV^3
    1.1*v^3 = V^3
    V = cube root of 1.1 * v
    V = 1.032*v
    That is a 3.2% increase in speed.
  6. Oct 23, 2009 #5
    Ah, I didn't realize I could take the cube root of 1.1 and v separately. thank you.
  7. Oct 23, 2009 #6


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    Ah, yes. And I didn't make it very clear, not having a cube root symbol handy. Most welcome!
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