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Why is Friction pointed Perpendicular to the Tires and not along the plane the tires are rotating

  1. Oct 31, 2014 #1
    So I know for a mass m with velocity v to travel in a circular motion, there has to be a centripetal force towards the center, but I'm having trouble intuitively visualizing why friction is directed towards the center?

    The way I see it, is for the car to turn , say left, the wheels rotate to the left, and as they're rotating they sort of "grab the ground and push back on it". As a result of Newtons 3rd law, the ground pushes back on the wheels, therefore wouldn't the force be acting in the plane the wheels are rotating, instead of perpendicular to them? I've attempted to show what direction friction acts on the tire.

    I know this has been asked several times on this thread, but after days of looking up different threads, I still can't get it to sink in, thanks I appreciate any help.
    Last edited: Oct 31, 2014
  2. jcsd
  3. Oct 31, 2014 #2
  4. Nov 1, 2014 #3


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    It's not exactly towards the center, if the wheels are for example driven by the engine via the axis.

    This sounds like you are talking about wheels driven by the engine via the axis. For simplicity you should rather consider
    steering with ideal free spinning wheels, which can only provide lateral resistance.

    Press a toy car firmly against a high friction surface, and try to move it sideways (perpendicular to the plane the wheels). The resistance the car offers is friction of the wheels perpendicular to the plane the wheels. That's the lateral resistance crucial for steering.

  5. Nov 1, 2014 #4
    Thanks so much for replying quickly, as this is my first question on here for a while, but i am a complete foreigner to your terminology/explanation. But after
    Hours of looking up threads here, i somewhat found luck off this thread
    https://www.physicsforums.com/threads/how-does-friction-causes-centripetal-acceleration.673274/. And the vector diagram provided by cepheid which shows a rolling friction and sliding friction being added to give the the friction that is radially inward and perpendicular to the tire .It looks like i had one part of the diagram in mine ( the reaction force from the tires rolling against the pavement) but i was missing the fact that the car still has some momentum in the direction of its velocity and thus the tires while rolling in the direction the tires were rotated are still sliding somewhat against the pavement and thus there is friction that opposes this sliding in the direction of the blue vector in the diagram
    Last edited: Nov 1, 2014
  6. Nov 1, 2014 #5


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    A car always has momentum only in the direction of its velocity.

    It has nothing to do with sliding. Wheels can provide lateral resistance without any slippage via static friction.
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