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Homework Help: Torque whatmakes a car accelerate

  1. Jul 14, 2006 #1


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    For a car that is accelerating to the right,
    I don't understand for if the tyre is rotating clockwise, the force is acting to the right.
    From the diagram, in the above web site, the direction of Ff and Fr are to the right.

    For example, if u have a door in front of you, fixed to an axis on the left hand side. http://www.cofc.edu/~agrestm/RS/Door.JPG
    Like the diagram shown, if you exert a force on the end , then the door will rotate anticlockwise. I could understand this phenomenon.

    I couldn't imagine why the force is acting on the tyre to the right, whereas the rotation is clockwise. I suppose the direction should be anticlockwise if the force acting on the tyre is to the right.

    I have been thinking too much, and need someone's else helps.

  2. jcsd
  3. Jul 14, 2006 #2


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    Staff Emeritus
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    If one is looking at the tyre, then rotation (caused by a torque) in the clockwise direction would produce motion at the bottom of the tyre in the left direction. The ground (road) applies an opposite force in the right direction, tangent to the tyre's circumference.

    So the tyre 'pushes' to the left and the car goes to the right in the opposite direction.
  4. Jul 20, 2006 #3


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    Thanks for your answer but I still couldn't get it. I'll think about it for some time and will get back to here then.
  5. Jul 20, 2006 #4

    Doc Al

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    Staff: Mentor

    Realize that while the engine makes the tires spin, if there were no friction from the road the car could not accelerate. The friction acts to prevent the tires from slipping; thus if the tires turn clockwise, friction must act to the right. Which should make sense, since the car accelerates to the right and friction is the only force providing the acceleration.
  6. Jul 28, 2006 #5


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    Now, I understand. Let me explain to myself.

    Suppose the car is of no contact with the road. If we turn on the engine, the tyre will spin, rotating clockwise at an angular speed e.g. w1.

    Now, imagine that we suddenly drop this car to the road, the angular speed of the tyre will become less than w1, which is e.g. w2. The decrease of angular speed is due to the torque provided by the friction of the road which is acting to the right. The decrease in rotational kinetic energy goes to the work done by the friction.

    Now, I can explain to myself why does the friction acts to the right which is the result of the decrease of angular speed.

    Haha, am I right? :rofl:
  7. Jul 28, 2006 #6


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    Thank you to Astronuc and Doc Al. Your answers add some ideas to my self-explanatin.
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