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Automotive Forces acted on the car when a car moves

  1. Jul 26, 2012 #1
    Hi everyone, good day.
    when a car is moving (accelerating), what are the forces that actually acted on the car?
    Is it possible to draw a free body diagram to illustrate the idea? (please draw one for me, i really need this in order to understand)
    I am having problem in relating how the engine torque causing the car to move forward. i mean i can totally understand how engine convert chemical energy in petroleum to rotating output shaft, and how output shaft goes through gearing to the wheel (i understand the physical mechanism). but i failed to draw a free body diagram to analyze the system.
    Also, i am quite confuse in the direction of friction acting on the car wheels. Say, the car is front wheel driven by engine, i heard people saying the friction force is the same direction as the car's forward motion on the front wheel (driven by engine), while is in the opposite direction with the car's forward motion on the back wheel (non driven wheel). is this true? if it is, can you please explain to me why it is so?
    Thanks a lot for you reply!
  2. jcsd
  3. Jul 26, 2012 #2


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    Can't you give the fbd a try yourself? Then we can help you fix the errors.
  4. Jul 26, 2012 #3
    Hi, russ_watters, thanks for the reply.
    attached is the fbd i tried. please comment on it. Thank you!
    F_f = friction act on front wheel
    F_b = friction act on rear wheel
    N_f = normal force act on front wheel
    N_b = normal force act on rear wheel
    w = weight of the car
    T = torque from the engine acted on front wheel

    Attached Files:

  5. Jul 26, 2012 #4
    Also, i am actually quite confuse on one thing. let say, there are two cars, all features on them are the same except one, the mass of two cars. Now, if the two cars are accelerated to the same speed and then the engine is turned off, they will slide for some distance and then come to stop.
    my question is, which car can slide further? and what theory explains the answer?
    originally, i am thinking of the lighter car can slide farther because the friction force act on it is lesser (smaller normal force), but i recalled that the body with higher mass has higher inertia, which maybe allows the heavier car to slide further.
    can anyone clarify this for me? this has been bugging me for few days. thanks a lot!
  6. Jul 27, 2012 #5

    Ranger Mike

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    read post on last page of Jul24-12, 10:34 AM

    Re: Race car suspension Class in this forum
  7. Jul 27, 2012 #6
    Hi Ranger Mike, I have read the article. It does not really answer my question. It does not clear my confusion. Can you please elaborate further?
  8. Jul 27, 2012 #7


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    What is your F_b force acting horizontally on the rear wheel? Aditionally, do you want to take into account wind resistance?
  9. Jul 27, 2012 #8

    jack action

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    Look at the theory at the bottom of this page (Theory »» Longitudinal acceleration).
  10. Jul 27, 2012 #9


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    The car has power to its front wheels.
    They start with no rotation and the motor torques them.
    If they were on a frictionless surface, what would the front wheels try to do? Which way would they rotate, if at all?

    The car has no power to its rear wheels.
    They start with no rotation and there's nothing to torque them.
    If they were on a frictionless surface what would the rear wheels try to do? Which way would they rotate, if at all?
  11. Jul 28, 2012 #10
    Hi, Mech_Engineer, no, i do not want to include wind resistance. the F_b is the friction acted on rear wheel.

    Hi, DaveC426913, the front wheel will skid on that frictionless surface, same direction with the torque.
    while for rear wheel, in my opinion, since there is no torque acted on them, they will not rotate. Am i right? that will be for the case of frictionless surface, i failed to extend the idea to the surface with friction, can you please give me more hints?
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