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fix air friction calculation #669

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fix air friction calculation #669

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Commits on Sep 24, 2018

  1. fix air friction calculation 

    The code is slghtliy confusing because `velocityPrevX` and
`velocityPrevY` actually define a distance. The real velocity can be
calculated as `velocityPrevX / deltaTimePrev` and `velocityPrev /
deltaTimePrev` where `deltaTimePrev = deltaTimeScaled / correction`.

This is the formular without air friction:

```
velocity.x = velocityPrevX * correction + body.force.x / body.mass * deltaTimeSquared
```

Air friction is a force that acts in the opposite direction of the
velocity. It is proportional to velocity (for small velocity):

```
velocity.x = velocityPrevX * correction + (body.force.x - (body.frictionAir * velocityPrevX / deltaTimeScaled * correction)) / body.mass * deltaTimeSquared
```

which can be converted to:

```
velocity.x = velocityPrevX * (1 - body.frictionAir * deltaTimeScaled / body.mass) * correction + body.force.x / body.mass * deltaTimeSquared
```
    @xi
    xi committed Sep 24, 2018
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