Comment 28614

By UrbanRenaissance (registered) | Posted February 12, 2009 at 08:56:27

Great article Ted! This is the kind of example I give any time anyone says "Oh I never really need math/science in my daily life." Most people don't realize that braking distance increases with the square of velocity, so doubling your speed actually quadruples your stopping distance. Incidentally, that equation to derive above is exactly the one police use to determine (roughly) how fast a car was moving based on its skid marks.

Though, the engineer in me does want to clarify your explanation of static vs. kinetic friction. For a rolling wheel (that isn't skidding) the static coefficient is used because at any given instant the part of the wheel touching the ground doesn't move relative to the ground. It briefly touches the ground with only the weight of the car pushing down, that piece of the tire has basically no forward or backwards movement. Once the wheel starts skidding however, the same part of the wheel slides along the ground, meaning in this case, the part of the wheel touching the ground is now moving relative to the ground, implying that the kinetic coefficient should be used.

This is why anti-lock brakes were created, to prevent skidding and decrease stopping distance by taking advantage of the higher coefficient of static friction. Although when its icy out your best protection is a set of snow tires and common sense!

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