Projectile Motion

Projectile motion is a special case of two-dimensional motion: in it, the horizontal speed is constant, but the vertical speed is subject to a constant acceleration:
```
         vx  =  const            ax  =  0
                                 ay  =  -g
```
As with other 2-D problems, one can separate the horizontal and vertical motions completely.
Any object thrown by hand (or by machine) through the air exhibits projectile motion: balls, bullets, cannonballs, rocks, etc.
One useful thing to remember: when an object reaches the peak of its trajectory, its vertical speed is zero.
Another useful thing to remember: if an object is thrown from some altitude, and then falls back to the same altitude, it takes the same amount of time to rise to its peak as it takes to fall back to the ground.

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