How does friction act on a car?

Answer 1

According to wikipedia friction is defined as: "the force resisting the relative motion of two surfaces in contact or a surface in contact with a fluid (e.g. air)." Thus there are would be 2 locations friction would occur on a moving vehicle: (1) air resisting the motion of the vehicle; (2) friction between the tire and the ground. Two types of friction can occur between the tire and the ground, a) static friction, and b) kinetic friction. Static friction is when the tire maintains grip or traction on the road surface, whereas kinetic friction (as the name implies) is when the tire is moving relative to the ground. To illustrate static friction, consider for a moment a dot on the tire's surface. Your vehicle is moving forward (let's say it's moving very slowly), and your tire rotates so that the dot comes into contact with the ground at a certain point. Since a car tire compresses a bit on the road's surface, there's approximately 3-4 inches of tire flat against the road at any given time. As your car moves forward, the tire rotates, and once the dot touches the ground at a given point, the tire and the ground move at the same rate relative to the car. That is, the dot on the tire and the point on the ground remain in contact until the tire reaches the end of that 3-4" strip of contact, when it is pulled upward from the ground to rotate around top and back to the ground. An example of kinetic friction from the illustration above would be that the dot on the tire reaches a point on the ground, but the dot and point move away from one another. In real life this would be if you hit the brakes and skid, or if you hit the gas and burn out or spin your tires (e.g. in the snow or mud). The problem with kinetic friction is that it is weaker than static friction. Thus, when you hit the brakes, if your tires lock up (you'll hear the squealing tires against the road) you are now in kinetic friction and your car will slow down less (actually speed up) compared to when your tires were in static friction with the ground. That is why you pump your brakes... also why anti-lock braking systems (ABS) were developed.