# Physics

##### Jun 22, 2014

### Billard ball physics

A few common formulas

```
velocity = change in distance / change in time
acceleration = change in velocity / change in time
force = mass * acceleration
momentum = mass * velocity
```

### Conservation of Momentum

The total momentum of a system before a collision is equal to the total momentum of a system after a collision.
**A** is Ball A. **B** is Ball B. **vx** is velocity x

```
A.vx = ((A.mass - B.mass) * A.vx + 2 * B.mass * B.vx) / (A.mass + B.mass)
```

If both objects have the same mass the formula simplifies.
We can think of it as **A** and **B** swap momentums.

```
A.vx = 2 * B.mass * B.vx / (A.mass + B.mass)
```

### atan2

`Math.atan2`

gives you the angle between two vectors.
Subtract the distance between each x and each y coordinate of the vector.

```
| .(2,4)
| /
| /
| /
| .(1,1)
+----------------+
Math.atan2(1, 3) * 180 / Math.PI
> 18.43494882292201
```

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