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So far in this book, we have ignored friction in order to make problems easier and to understand the basic principles behind motion. On the other hand, there cannot be too many practical applications of such a theory without friction, since few of the surfaces in this world are frictionless, and it is difficult to go anywhere without air resistance (especially if you go by car).

Friction is a force that opposes the slipping of two surfaces, so it acts parallel to the boundary between the surfaces. We generally think of friction as the force that slows things down, but that is not the best way to consider it. When you step on the accelerator of a car, what force makes the car go faster? None other than the friction between the tires and the road. What happens when you try to accelerate on ice?

There are two types of friction: static friction, which is relevant when the surfaces are not slipping, and kinetic friction, which is relevant when the surfaces are slipping.

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