# car pileup

## Slip-Slidin' Away

Good thing the cars in this video are all moving slowly. Add a little more speed, and the scene would be a driver’s worst nightmare. Imagine a car pileup in front of you on a snowy day, your own skidding wheels and, seconds later, the inevitable crash…

Consider—the reason people can control their cars is that it’s very hard to slide a tire across pavement. Technically speaking, this is because tires are built to have a high coefficient of friction when pressed on a paved road. The coefficient of friction is essentially a ratio of the force it takes to slide two surfaces across each other to the force they’re being pressed together with. A high coefficient of friction means the two surfaces don’t like to slide; a low coefficient of friction means it’s easy. For example, let’s say you’re speeding down the highway and you see a police officer, so you step on your brakes. The amount of force it would take for your car to skid is the weight of your car (the force pressing the car to the road) multiplied by the coefficient of friction. When the pavement is dry, the coefficient of friction is high, so you can apply a lot of braking force without skidding.

On the fateful snowy day in our video, things worked a little differently. When these people pressed the brakes, the heat generated by the tire-on-ice friction created a thin film of water over the frozen surface. The coefficient of friction for tires on ice with a thin film of water between them is pretty much zip, resulting in—you guessed it—auto Ice Capades. It took almost no braking force for the cars to skid and, once skidding, they continued in a uniform motion, on a decline, until they found something that could apply enough force to stop them. The most convenient thing, as it all too often is, was another car.

There’s not a whole lot you can do in a situation like this besides try to steer out of the line of other cars and gently brake in the hopes that your antilock system helps the wheels grip again. What didn’t seem to work was when one guy jumped out of his car, grabbed the door, and tried to stop it himself. Maybe he can bench-press a few, but it’s doubtful he could have competed against the villainous combination of ice, rubber and a low coefficient of friction. —Katherine Ryder

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### June 2013: American Energy Independence

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