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Homework Help: Given equation for position with unkown constant, find the constant for given speed

  1. Sep 27, 2008 #1
    1. The problem statement, all variables and given/known data

    A car in a roller coaster moves along a track that consists of a sequence of ups and downs. Let the x axis be parallel to the ground and the positive y axis point upward. In the time interval from t=0 to t=4 s, the trajectory of the car along a certain section of the track is given by

    r = At(xhat) + A(t^3 - 6t^2)(y hat)

    where A is a positive dimensionless constant.

    A) Derive a general expression for the speed v of the car.

    The roller coaster is designed according to safety regulations that prohibit the speed of the car from exceeding 20. Find the maximum value of A allowed by these regulations.

    2. Relevant equations

    r = At(xhat) + A(t^3 - 6t^2)(y hat)

    3. The attempt at a solution

    I think I can do the first part properly. I found the derivative of the xhat equation and the y hat equation, and then found the magnitude of speed using c^2 = a^2 + b^2 of the derivatives, which comes out to

    v = sqrt [A^2 + (A (3t^2 - 12t))^2]

    or simplified

    v = A * sqrt[1 + 9t^2 * (t-4)^2]

    THe program tells me this answer is correct.

    I don't know how the do the last part of the equation at all. A is an unknown, but t is a variable as well....I tried using a hint in the program but it just tels me the answer should be vmax = A * sqrt [145]

    I thought maybe, you find the derivative of v(x) and check when it's zero, then you know it's maximum velocity. But in the answer given, the derivative of v(x) is supposed to have v in the denominator, and I don't see how they got that answer....and then what? Where did the t go to get that final v max?
  2. jcsd
  3. Sep 27, 2008 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi theowne! Welcome to PF! :smile:

    Hint: you're only asked for 0≤t≤4 …

    the maximum may be at dv/dt = 0, or it may be at 0 or 4 :wink:

    EDIT: hmm … just noticed :redface: … you can see where the maximum is without any hard work …

    v is a maximum when t(t - 4) is a maximum, isn't it? :smile:
    Last edited: Sep 27, 2008
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