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Car Jack Torque problem

  1. Feb 6, 2005 #1
    A mechanic jacks up a car to an angle of 8 degress to change the front tires. The car is 3.05 m long and has a mass of 1130 kg. Its center of mass is located 1.12 m from the front end. The rear wheels are .4 m from the back end. Calculate the torque exerted by the car around the back weels.

    The answer is 1.68 times 10 to the 4

    But i can't figure out how to get the to the answer. I'm not sure where i can put the pivot point... the distance of the jack is unavailable. this problem is keeping me from doing any other problems...help.
  2. jcsd
  3. Feb 7, 2005 #2
    The first thing to do is draw a picture of the problem.
  4. Feb 7, 2005 #3


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    You can determine the point of pivot by the thought experiment "What would happen if the jack were to sudently disapear? Namely, around what point in the car would the car "rotate"? Or even, what is the only point in the car that would remain still after the jack is removed?"

    The mention of the jack is superfial; it is only to give a context to the problem.
  5. Feb 7, 2005 #4


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    1) You'll have to assume that the jack is at the very front of the car.
    2) There is only one force acting. However, you are interested in one of the components of that force. Which one should you be looking at?
    3) The pivot point is the rear wheels.
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