Obtain Quicktime 3.0 from Apple

No Air Friction
This projectile is moving in a typical parabolic path.  The horizontal velocity remains constant (notice the constant spacing).  The vertical motion is influenced only by gravity.  The initial velocity and direction of the projectile are enough to have the projectile go over the mountain.

Obtain Quicktime 3.0 from Apple

With Air Friction
This projectile was fired at the same velocity and direction as the one above.   Notice that the path is no longer parabolic.  The projectile does not travel with a constant horizontal velocity; it decelerates.  The vertical motion is now under the combined influence of gravity and air friction.  They work together on the way up, but work against each other on the way down.  Gravity works only downward, while air friction works against the direction of motion of the object.  The projectile in unable to clear the mountain.

Air friction can rarely be removed but for fairly massive projectiles over small times and distances, the effects are typically small.  Take this basketball shot as an example.  Notice the horizontal velocity of the ball is fairly consistent.  You can tell this by looking at the spacing of the vertical lines which mark the position of the ball every 10th of a second.  The spacing is quite consistent, which indicates a constant horizontal velocity. 

If the basketball were less massive, like a crumpled up piece of paper, we would notice the vertical lines would get closer together indicating that the ball was slowing down.