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.