Acceleration

Freefall describes any motion in which the earth's gravity is the only force acting on an object.
In freefall, a body experiences no horizontal acceleration, but a constant acceleration of 9.8 m/s^2 towards the earth's center -- that is, down. We define a constant, called "gee",
```
          g  =  9.8 m/s^2
```
which is the magnitude of acceleration due to earth's gravity. Thus, for most objects in flight,
```
          ax  =    0        horizontally
          ay  =   -g        vertically
```
Airbags save lives simply by increasing the distance over which human bodies decelerate as a car crashes.

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