|MadSci Network: Engineering|
To understand why road curves are banked requires a little education on friction. When you slide a box on the ground, the amount of force it takes to push the box is dependant on the weight of the box and how slippery the surface is. This is because all surfaces exert what we call friction when they rub against each other. Pushing a heavy box requires more effort than a lighter weight box. If the surface is smooth, like a tile floor, it will be easier to push than on a rough floor, like a carpet. The amount of force required is equal to the weight of the box times the coeficient of friction, which is an engineering way to describe how slippery a surface is. The coeficient of friction of teflon, which is very slippery, is .1 or lower. The coeficient of rubber, which is what we make car tires from, can be greater than .5 So pushing a 100 lb box accross teflon would take 100 lb * 0.1=10 lb. Pushing that same 100 lb box across rubber would take 100 lb * 0.5 = 50 lb. If you try to push the box up a slope, it gets harder to push. That is because, in addition to pushing against the friction, you are also pushing against part of the weight of the box itself. If you imagine an arrow coming from the center of the box and pointing straight down towards the ground, that is the force of gravity pulling on the box. When the box is on an incline, most of the weight pushes against the incline, but a small portion pushes down the incline, trying to make the box slide down the slope. When something is made to travel in a circle, like a rock spun on the end of a string, it takes a force to keep it traveling in a circle. IF you spin a rock at the end of a string, the string pulls the rock and keeps it from flying off into the distance. The amount of force needed depends on how heavy the rock is and how fast you spin it. When you drive a car around a curve, the same principle applies. The car wants to go straight, so something must push on the car towards the center of the curve. Otherwise the car will go off the outside of the curve. If the roadway is flat, the force of the friction of the tires on the road push in the direction of the inside of the curve to keep it on the road. (The same force you feel when trying to push the box across the floor.) If you drive too fast, or the road is slippery from rain (low friction), the car can lose traction and will slide to the outside of the curve. If the roadway is banked, the car is like that box on the incline. The weight of the car will try to slide it down the incline. But that is in the same direction the force needed to keep it going in a curve must push. So the banked curve allows the weight of the car help keep the car from sliding off the road. The steeper the slope, the faster you can go without worrying about the tires losing traction and letting the car go off the road. Many fast race tracks like Daytona Speedway are banked very steeply to allow cars to go over 200 mph and still go around a curve safely.
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