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Free body diagram for each vehicle

  1. Jul 26, 2007 #1
    1. The problem statement, all variables and given/known data
    In the movie "Jurassic Park 2", a jeep is struggling to pull a trailer back up from over a cliff. The jeep is connected to the trailer by a cable. The jeep in on level ground and the trailer hangs from the cable over the of the cliff. The jeep has a mass of 850 kg and pulls with a force of 21,000 N. The trailer has a mass of 2150 kg. Ignore friction

    2. Relevant equations

    3. The attempt at a solution
    I drew the picture and made a free body diagram. The directions told me to make a free body diagram for each vehicle. It is attached.

    a = m[tex]_{b}[/tex]g/m[tex]_{a}[/tex] + m[tex]_{b}[/tex]

    The force that jeep is pulling with a is throwing me off. The answer to the problem is -0.023 m/s[tex]^{2}[/tex] I just want to know the steps to get to the answer.

    Attached Files:

  2. jcsd
  3. Jul 26, 2007 #2

    Doc Al

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    Staff: Mentor

    Your diagrams neglect the tension in the cable, which exerts a force on both vehicles.

    You will apply Newton's 2nd law to each object, realizing that they are connected and thus must have the same magnitude of acceleration. You'll get two equations, which you can solve together to get the acceleration.
  4. Jul 26, 2007 #3
    got it.

    Ftrailer = 2150kg(-9.8m/s^2)
    Ftrailer = 21,070N

    Fresultant = FJeep + Ftrailer
    Fresultant = 21,000N -21070N
    Rresultant = -70N

    -70 N = (850kg + 2150kg)a

    a=0.023 m/s^2
  5. Jul 26, 2007 #4


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    Science Advisor
    Homework Helper
    Gold Member

    You get the right answer doing it this way, but your method is shaky. You take a horizontal force of the jeep and add it to the vertical weight of the trailer and end up with a resultant by simple addition? You should instead practice drawing good Free Body Diagrams of each object, as Doc Al suggested. Otherwise, sooner or later you're going to get burnt doing it this way.
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