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Cable car with 2 cable arrangement

  1. Nov 13, 2016 #1
    1. The problem statement, all variables and given/known data
    The century-old ascensores in Valparaiso, Chile, are small cable cars that go up and down the steep hillsides. As the figure(Figure 1) shows, one car ascends as the other descends. The cars use a two-cable arrangement to compensate for friction; one cable passing around a large pulley connects the cars, the second is pulled by a small motor. Suppose the mass of both cars (with passengers) is 1600 kg , the coefficient of rolling friction is 3.0×10−2, and the cars move at constant speed.

    What is the tension in the connecting cable?
    What is the tension in the cable to the motor?
    https://session.masteringphysics.com/problemAsset/1383182/3/07.P43.jpg

    2. Relevant equations
    F=ma,since a=0, Fnet=0

    3. The attempt at a solution
    I drew a free body diagram on both of the cable cars, since F net=0, I was assuming that the x component of all the forces are 0, which are the rolling friction and the x component of the gravitational force. I also assumed that since both cars are 1600kg one's mass would be 800kg? with the rolling friction provided I thought the answer would be 0.03 cos 35 degree*m*g+sin 35 degree*m*g=T, but it seems to be incorrect.
    Also I was just completely confused on the effect of the cable between the motor and one of the car. Does it pull the car up? although it appears on the picture that gravity from one car was pulling the other on up?
    Thank you.
     
  2. jcsd
  3. Nov 13, 2016 #2

    kuruman

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    Look at it this way. If there were no friction, the cars would be completely balanced and a small nudge would get them moving one way or the other. With friction, the pull of the motor is needed to overcome just the friction but not gravity. It might be easier to see what's going on if you drew separate FBDs for the two cars.
     
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