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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