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Direction of friction in the circular motion of a car

  1. Apr 4, 2013 #1
    1. The problem statement, all variables and given/known data
    Why does friction act towards the center when a car does uniform circular motion by itself.




    3. The attempt at a solution

    I know that centripetal force is needed to move an object in a centre and so there should be a force acting towards the center in the circular motion of the car. But I had also read that friction opposes the relative motion of 2 surfaces and so I think that friction should always act opposite to the tangential velocity of the car. For example, when i move a car linearly the friction opposes its linear velocity. Please help me and tell me why does friction act towards the center?
     
  2. jcsd
  3. Apr 4, 2013 #2

    mfb

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    Staff: Mentor

    That is sliding friction - and even there, it is just a useful approximation. Static friction can act in different directions, too.

    Using the same example, you can use static friction to accelerate a car (that is how cars usually accelerate).
     
  4. Apr 4, 2013 #3
    Can you please explain a bit more. What type of friction is this? i know only 2 types of friction -static and dynamic...
     
  5. Apr 4, 2013 #4
    you need to turn the front wheels to turn.
    What will this do to the direction of the friction force?
    (On a 'banked' track you do not need to turn the front wheels, how does the centripetal force arise in this case?)
     
  6. Apr 4, 2013 #5

    mfb

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    sliding = kinetic = dynamic friction
     
  7. Apr 4, 2013 #6
    On a banked track, I think that the horizontal component of the weight of the car will act as the centripetal force. I think that when the wheels of the car are turned the force which pushes the car will tend to make the wheels skid and so maybe the friction will act in the center to prevent the sliding motion of the wheel and provide the centripetal force for the motion of the car. I think that when the wheel of the car is in the turned state, the force on the car would have 2 componants, one along the wheel and one perpendicular to it. Would the friction also cancel the perpendicular component in addition to providing the centripetal force? Am I correct?
     
  8. Apr 4, 2013 #7
    you have produced a pretty good description of the important points.
    One small criticism....the WEIGHT of the car does not have a horizontal component !! It is the force of the banked track on the car that produces the horizontal component that is the centripetal force.
    Can you draw a force diagram to show the forces acting on the car?
     
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