In designing an object, it is often better to use a shape that is simpler than, though similar to, the particular final shape you had in mind. This will allow for easier, faster collision detection, which might be worth the trade-off of a specific complex shape.
Creating methods on the game object itself, rather than in the timeline, is a good programming practice that helps keep the process flexible and not tied to graphics on the screen.
The variable decay allows you to control the slowdown of an object by lowering the magnitude of its speed by a percentage each time a collision with a wall or floor occurs. This allows a bouncing ball in Flash to act more like a ball would in real life; it eventually stops bouncing.
Momentum is a vector which is stated mathematically as momentum = mass*velocity. The momentum of an object (or system) is conserved if the total external force on it is 0.
Energy is the measure of a system's ability to do work. Kinetic energy is associated with the movement of an object. Potential energy is stored in an object, and can be converted to kinetic energy (a ball sitting on the roof of a house has potential energy that is converted to kinetic energy when it falls). Energy is conserved when the final energy in a collision is equal to the initial energy.
The quantities of momentum or of energy do not change in the course of a collision in a closed system (that is, one with no net external forces). Think of hitting a cue ball dead-on into another ball and seeing the cue ball sit perfectly still while the other moves away.
Elastic collisions are those in which both kinetic energy and momentum are conserved.
When two objects collide, the momentum that is affected is the component that lies along the line of action.