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Car forces

  1. Nov 27, 2008 #1
    I am building a project,where you drive a car on a straight road.

    The car has 3 kinds of forces:
    1)Torque of engine in the wheels
    2)Friction on the tyres
    3)Force due to air resistance

    If a car accelerating,and is front wheel drive,then the 2 front wheels have these forces:

    <-------(Engine Torque)-------O------(Static friction)-------------->

    and the rear wheels

    <-------(Rolling friction)-------O

    so car has these forces:

    <---(Engine torque,rolling friction,air resistance force)--(CAR)--(Static friction force)---->

    So,Whats the total force?

    F=engine_torque-air_resistance-rolling_friction-static_friction ?????

    I read that static friction causes movement...so static friction is equal to engine torque?I am comfused.
  2. jcsd
  3. Nov 27, 2008 #2


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    I think it would make the problem more clear if you think of engine torque and static friction being in the same direction.

    <---(Engine torque ~ Static friction force)--(CAR)--(rolling friction, air resistance)---->
  4. Nov 27, 2008 #3
    so,if engine torque on front wheels<=static friction then


    else ,if engine torque on front wheels>static friction then


    When car speed=0,what is the minimum engine torque to start the car to move?

    where x is friction resistance on the front wheels.

    What is the x?rolling friction again?
  5. Nov 27, 2008 #4


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    It might also be a good idea to stop referring to the force as static friction.
    Although technically correct, it's a bit confusing, as evidenced by your following statement:
    If the torque exceeds the static friction then you no longer have static friction, ie. your wheels will spin and you will have kinetic friction.
    It would be equal to the force required to move the car.
    Rolling friction is the resistance of a wheel being rolled over a surface.
  6. Nov 27, 2008 #5
    I cant understand what is the minimun engine torque required to move the car,if initialy car speed=0..there is no resistance?

    when car speed is >0 then the rear wheels have rolling friction,and the front wheels static friction so

    if speed>0 then

    else is speed =0 then

    Is this right?
  7. Nov 27, 2008 #6


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    There is always resistance.
    The front wheels also have rolling friction.
    Basically. Yes.
    I do not understand the question.
  8. Nov 28, 2008 #7
    then,if and front wheels and rear wheels have rolling resistance,when speed>0


    if speed=0,which is the minimum engine torque required to start the car to move?

    Why dont you answer my questions?

    I need to know which are the forces in the car when speed>0 and which is the minimun
    engine torque required to move the car if speed=0.

    There are 4 kinds of forces:
    1)engine torque
    2)rear_wheels rolling resistance
    3)front wheels rolling resistance
    4)air resistance

    Give me an answer please...
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