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Homework Help: An accelerating car

  1. Nov 1, 2012 #1
    1. The problem statement, all variables and given/known data
    When the wheels rotate backwards,a backward force is exerted on the ground by the tyres.
    By 3rd law,a equal but opposite force acts on the tyre by the ground,so the car
    But I have also found some websites that states static friction is the only external
    force causing the car to move forwards.
    In some typical physics questions,the driving force and friction are two different
    forces act on a car,where friction is in the opposite direction of displacement.
    I would like to ask which force is actually accelerating the car?Friction or reaction
    force by the ground(due to the engine)?
    If it is friction,why backward arrow is drawn in some textbooks to represent friction while
    the car is accelerating forwards?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 1, 2012 #2


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    Science Advisor
    Homework Helper
    Gold Member

    yes, this is a friction force
    yes, by friction
    that is correct
    the driving force (or traction force) is the friction force on the driving tyres acting forward; friction on the non driving tyres acts backwards
    friction is the reaction force, accelerating the car
    maybe they are showing the forces acting on the ground by the tyres and not the force on the tyres from the ground..?
  4. Nov 1, 2012 #3
    That's probably showing the other friction forces like air resistance and rolling friction that are tending to slow the car down and so act backwards.

    That's separate from the reactionary friction force on the wheels driving the car forward.
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