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Force applied perpendicular to velocity, kinetic or static friction?

  1. Sep 28, 2011 #1
    The other day, we were discussing cars going through turns and the max speed they could do without slipping. My teacher asked us if static or kinetic friction should be used. I answered static, because the wheels are rotating and thus not sliding over the road surface. He ended up saying the answer was static. However, I don't agree with his reason. He said, the car was moving in one direction and the force was being applied perpendicular to that direction it, and thus the car wasn't moving in that direction. Since it wasn't moving relative to the force, static friction would apply.

    This leads me to two questions:
    Static friction applies to normal (nonskidding) forward motion of a car's tires, right?

    A cube is moving at a constant speed of 1 in the x direction, and 0 in the y direction. A force of 1 is applied in the y direction. This would be kinetic friction, right?

    I would think it's static because of the (admittedly crude) understanding I have of why static vs kinetic friction applies. As in, tiny bumps on the two surfaces lock up when the object is not moving. When it is moving, the two surfaces glide over each other, the bumps still hitting each other and slowing it down, but not as much as when they could completely lock. It would seem that it wouldn't matter if the object then began moving perpendicular too. The bumps would still be moving over each other. Perhaps the real answer is more complicated and it would be something between kinetic and static friction.
  2. jcsd
  3. Sep 29, 2011 #2


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    If a car is accelerating without the tires slipping, then the force is in the same direction as the velocity, but it's still static friction. As you mentioned, if there's no sliding involved, then it's static friction.
  4. Sep 29, 2011 #3
    Yeah I don't agree with teacher's logic either, as it would not hold up to a car braking or accelerating. A braking or accelerating car (not skidding) will still use static friction despite the force being applied in the same direction as motion. As long as there's no slipping, then it's static.

    I have ran into many times where my physics/ engineering professor was straight up wrong. Word of advice, feel free to ask "is that really true", but don't push it if he sticks to his answer. You WILL end up being wrong, whether you are truly wrong or not.
  5. Sep 29, 2011 #4
    I think this is a case of 'no slip' as already noted by others so the friction coefficient is static.

    However your cube is not the same as car tyres.

    Wheels are subject to static friction in the direction of motion because the point of contact is not moving, due to the rolling action of the wheel.

    This is an idealisation as real tyre/road interactions are very complex.
  6. Sep 29, 2011 #5

    Philip Wood

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    The cube case (sliding, not rolling) interests me...

    You won't, of course get a sideways friction force unless there's another, opposite, sideways external force on the cube. I'd have thought that, bearing in mind that the block is in motion ('forwards'), whether or not the block then slips sideways is more related to dynamic friction than static, for the very reason you (the OP) gave originally...
    My take is purely speculative and carries no authority whatsoever. I've no special knowledge about frictional forces, except the knowledge that they are complicated, the results are hard to reproduce, and the so-called Laws of Friction are approximate.
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