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Friction BasicsFriction is a force that holds back the movement of an object. That's it. Friction is just that simple. You will find friction everywhere that objects come into contact with each other. The force acts in an opposite direction to the way an object wants to move. If a car needs to stop at a stop sign, it slows because of the friction between the brakes and the wheels. If you run down the sidewalk and stop quickly, you can stop because of the friction between your shoes and the cement.What happens if you run down the sidewalk and you try to stop on an oil puddle? Friction is still there, but because of the oil there's a lot less. Less friction means it is harder to stop. The low friction thing happens to cars when it rains. That's why there are often so many accidents. Even though the friction of the brakes is still there, the wheels are not in as much contact with the ground. Cars hydroplane when they go too fast on very shallow puddles of oily water. 
Friction and GasesWith our examples, you might be thinking friction happens with solid objects. Nope. You just need matter and any state will do. Even a gas can cause friction. Friction in the air is called wind resistance.If you were in the space shuttle and re-entering the atmosphere, the bottom of the shuttle would be getting very hot. The friction between the molecules of air surrounding the shuttle creates that heat. As the molecules rush against each other at super high speeds and pressures, the bottom of the spacecraft heats up. The temperature on the top of the shuttle is also warm, but nowhere near the temperatures found on the bottom. Friction and LiquidsFriction can also be found in liquids. Scientists call the friction in liquids "viscous forces" or "viscosity." Yes, that's like the viscosity of the oil you put in your car. Car engines have a lot of moving parts and they rub on each other. The rubbing is friction and the result is heat. When oil is added to a car engine the oil helps to decrease the amount of friction and wear on the parts of the engine.
Measuring FrictionMeasures of friction are based on the type of materials being observed. Concrete on concrete has a very high coefficient of friction. That coefficient is a measure of how easily one object moves in relationship to another. When you have a high coefficient of friction, you have a lot of friction and interaction between the materials. Concrete has a very high coefficient and Teflon has a very low coefficient. Teflon is used on many surfaces when they don't want objects to stick such as pots and pans.Scientists have discovered that there is even less friction in your joints than in Teflon. It is one more example at how efficient living organisms can be. 
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