Physics is the study of the behavior of objects in nature on the most fundamental level.
Understanding basic physical laws and how to apply them is the key to creating dynamic realism in games.
Velocity is a vector formed by combining speed with a direction.
Acceleration also a vector occurs whenever the velocity changes.
If you know the acceleration of an object and its current velocity, you can find or project the velocity of that object at any time in the future.
While in the real world we generally think of speed as being measured in units of distance/time, in Flash we think of speed as being measured by frames. So Flash users usually assume one frame to be one unit of time.
When looking for the balance between creating smooth-appearing motion and not overtaxing the processors of most computers, 24 frames per second seems to offer the best results.
The amazingly simple trick for applying "good-enough" gravity to your effects is to come up with a value for gravity and add that value to your y velocity in every frame.
A frictional force is one that opposes the direction of motion and is caused by the interaction between two materials in other words, it slows something down.
Kinetic energy the energy associated with the momentum of an object is lost as heat from the friction, and the object slows down.
"Real" frictional implementation decreases the velocity linearly; "good-enough" friction decreases the velocity by a percentage of the current velocity (nonlinearly).
In most circumstances, the difference between these is not going to be worth the amount of coding you'd have to put into the ActionScript to arrive at the "correct" implementation.