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Homework Help: Test of a Car

  1. Jan 27, 2008 #1
    1. The problem statement, all variables and given/known data
    You have to find the value of k in the drag equation; Fdrag = k*v^2

    It is for a car driving. The motors maximal effect is P = 60W, and it uses this, when the car drives 120 km/h or above.

    Previously, I have calculated that between 30 km/h and 120 km/h, the resulting effect is P = 57W. How to calculate the k-value with these two values?
    The mass of the car is 1430 kg.

    If it helps, there is also given a tabel of the time and velocity:
    t: 0 2,2 3,1 5,5 8,9 13,3 18,9 (s)
    v: 0 30 40 60 80 100 120 (km/h)

    However, this was for one of the previous questions, not sure if it ought to be used.

    2. Relevant equations
    Maybe F*v = P ?

    3. The attempt at a solution
    I think I have to focuse on the time, where v = 120km/t. Here I know v and I know the two P's.
    So I though of an equation like this:

    60W - k*v^2*v = 57 W

    However, this gives a very strange k, so I think I am wrong. :(
  2. jcsd
  3. Jan 28, 2008 #2
    No ideas? :(
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