# Chapter 18: N5) Friction


• 18.1: Friction (Part 1)
When a body is in motion, it has resistance because the body interacts with its surroundings. This resistance is a force of friction. Friction opposes relative motion between systems in contact but also allows us to move, a concept that becomes obvious if you try to walk on ice. Friction is a common yet complex force, and its behavior still not completely understood. Still, it is possible to understand the circumstances in which it behaves.
• 18.2: Friction (Part 2)
Simple friction is always proportional to the normal force. When an object is not on a horizontal surface, as with an inclined plane, the force acting on the object that is directed perpendicular to the surface needs to be found.
• 18.3: More Examples

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