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Kinetic friction in an N,T- system only works in tangential?

  1. Jul 16, 2016 #1
    1. The problem statement, all variables and given/known data
    media%2Ff29%2Ff290f5b4-f833-410d-ad2a-4923fad01958%2FphpKQ6xmj.png

    2. Relevant equations
    F=m*a
    ΣF(n-direction)= m*a(n)= m*(v^2/ρ)
    ∑F(t-direction)= m*a(t)\
    Ffriction= Fn*coefficient(of friction)

    3. The attempt at a solution
    So I tried solving this question and apperently it is way easier than I thought.
    So I thought the kinetic friction has a component in the tangential direction of motion and one in the normall direction (pointing towards center or away from center).
    But apperently the kinetic friction in this question only points in tangential direction, but why?

    I tried rationalizing it, by saying friction only works in the direction of motion, but that is not true.
    Since a car that goes around a curved path, will notice that it's friction on it's tires will make him be able to take the corner/curve.
    Or is it that kinectic friction can only be in direction of motion and that the only friction that can work normall, towards center of curv, is the static component (unless u slipp away or towards center)?
     
  2. jcsd
  3. Jul 16, 2016 #2

    haruspex

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    Kinetic friction acts to oppose actual relative motion of the surfaces in contact. Static friction acts to oppose potential relative motion, i.e. the motion that would occur in the absence of friction.
    For a car cornering without skidding, at constant speed, the potential relative motion is radial. (Note, this relative motion of the road and surface of the rotating tyre, not relative motion of road and car.). Hence the static friction acts radially. If the car is also accelerating tangentially, by braking perhaps, then the friction will also have a tangential component. This is why braking on a bend can cause a skid. The tangential component reduces the available radial component.

    In the collar and ring question, the relative motion is tangential.
     
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