## Friction

• When a situation involves forces or motion along some direction, it often simplifies the analysis to tilt the coordinate axes so that one of them is parallel to that special direction.
• Friction is a force between two surfaces which opposes motion parallel to the interface.
• Friction depends upon two things:
1. properties of the two surfaces (wet/dry, rough/smooth, etc.)
2. the normal force between the two surfaces
• If two surfaces are not moving relative to each other, then static friction opposes the start of any motion. It has a size
```     F(static)  =  (normal force) * (coeff of static friction)
```
• If two surfaces are moving relative to each other, then kinetic friction opposes that motion. It has size
```     F(kinetic) =  (normal force) * (coeff of kinetic friction)
```
• In most cases, the coefficient of static friction is larger than the coefficient of kinetic friction; that is, it's easier to keep an object moving than to get it started from rest.

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