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Homework Help: Torque of a Stationary Car

  1. May 4, 2009 #1
    1. The problem statement, all variables and given/known data
    A mechanic jacks up a car to an angle of 10.0° to change the front tires. The car is 2.95 m long and has a mass of 1150 kg. Its center of mass is located 1.12 m from the front end. The rear wheels are 0.40 m from the back end. Calculate the torque exerted by the car around the back wheels.

    2. Relevant equations
    T = F x r x sintheta
    sum of the torques = 0
    F = ma

    3. The attempt at a solution
    Clueless when I have to figure out where the gravity is acting, and so forth. Basically, I don't know where to start.
  2. jcsd
  3. May 4, 2009 #2


    User Avatar
    Homework Helper

    Welcome to PF.

    Start by drawing a diagram.

    You are going to use the back wheel at the pivot point. And the sum of the Torques are going to be 0.

    But the question only looks like it wants the torque from the car, not the jack. So ... how far is the center of mass from the pivot along the horizontal after taking into account the angle the car is tilted at?

    That's about all you need to answer it.
  4. May 4, 2009 #3
    So you subtract the lengths to get from the jack to the rear wheels?
  5. May 4, 2009 #4
    Okay. I think I got it...part of my problem was that I didn't have my calculator set in degrees. The other part of my problem was that I had originally gotten the torque from the jack, not from the car.

    Thank you for your help!
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