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Rollercoaster (finding speed) test tomorrow, please help

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data

    Figure 4 shows a partial profile of a roller-coaster ride. Friction and air resistance are negligible. Determine the speed of the coaster at position C if the speed at the top of the highest hill is (a) zero and (b) 5.00m/s


    Figure 4 (description)

    Point A is the highest point of the rollercoaster. It is 37.8m from the ground. Point B is the lowest point of the rollercoaster. It is 12.8m from the ground. Point C is the mid point of the roller-coaster. It is 17.8m from the ground.

    *There are no loops
    *From A, it curves downwards to B, then upwards to C

    I apologize for not having a picture of this.

    2. Relevant equations
    E = 0.5mv^2
    E = mgh

    (sorry, not sure)


    3. The attempt at a solution

    I have no idea how to start this out...

    E = mgh
    = m x 9.8m/s^2 x 37.8m *keeping mass as a variable since mass is not given
    = 370.44(mass)J

    E = 0.5mv^2
    370.44(mass)J = 0.5mv^2 *masses cancel out
    square root of (370.44J / 0.5) = v
    27.22m/s = v

    The answer from the textbook: (a) 19.8m/s (b) 20.4m/s

    I don't even know how to start out this problem. :confused: Help please. Thanks in advance! :smile:

    EDIT:

    for (a) h = 37.8m - 17.8m = 20m

    Eg = mgh
    = mass x 9.8m/s^2 x 20m
    = 196 (mass) J

    Ek = 0.5mv^2
    196 (mass) J = 0.5mass x v^2 *masses cancel out
    Square root: (196J/0.5) = v <- forgot to divide the gravitational potential energy by 0.5
    19.8m/s = v

    Got the answer for (a) but don't know how to figure out (b)...
     
    Last edited: Oct 30, 2008
  2. jcsd
  3. Oct 30, 2008 #2
    bump =)

    still trying to figure out (b)
     
  4. Oct 30, 2008 #3

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    For part a, you set the initial energy (all potential) equal to the final energy (all kinetic). For b, the initial energy is part kinetic and part potential. Calculate it, and set it equal to the final energy to solve for v.
     
  5. Oct 30, 2008 #4
    Ah thank you very much good sir =)

    (a) Eg = Ek

    (b) Eg + Ek = Ek'
     
  6. Oct 23, 2010 #5
    How did you solve for part B using Eg + Ek = Ek'?
     
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