Free body diagram for each vehicle

  • #1

Homework Statement


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


Homework Equations





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.
Tried

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.
 

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Answers and Replies

  • #2
Doc Al
Mentor
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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.
 
  • #3
got it.

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

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

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

a=0.023 m/s^2
 
  • #4
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,161
504
got it.

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

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

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

a=0.023 m/s^2
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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