Sorry to butt in on your Sikorsky Gearbox thread Lu, but an answer to FS: I know you have not yet finished, so please forgive me for interjecting.
Your van model is at the critical stage for point 14. As the back of the van now applies the full acceleration to you, it is acting in a certain direction (in this case, in the same direction as the van is going). But all you feel a "force" pushing you back into the back doors of the van. This is not a force however, it is the reaction to the acceleration the van doors are attempting to place on your body. Proof of this is the fact that assuming a frictionless rollerskate/floor interface, you would have not moved from over the same point on the road until hit by the van doors, i.e. the acceleration needs to overcome your inertia. Your inertia does not apply a force to the doors. Here we are exploring Newtons action/reaction. The reaction is the resistance of your body to the acceleration (i.e. its inertia), but it is not a "force", it is a reaction to the force applied by the van doors in trying to make you change speed.
I think, however, that you would argue that your body would be placing a force on the doors, and it would certainly feel like that. Using your previous formula of F=ma, you are the mass, and the acceleration is provided by the doors. Getting back to the critical issue of direction now (i.e. which way does the force act), as TeeS quite rightly points out, acceleration is not a force per se, it is more correct to say that it becomes a force when it acts upon a mass (F=ma as you pointed out). But acceleration has a direction. Mass doesn't. Therefore force must have a direction because it is a product of acceleration, and certainly the convention of vector analysis would depict that direction. Thus the direction of the force is said to be acting in the direction of the acceleration, Hence the validity of point 14.
To TeeS: you are technically correct, appologies for any confusion. I have answered your acceleration/force relationship above. As for speed Vs direction, I believe that I was probably trying to say "velocity" and express it as a vector, and have (incorrectly) over simplified the explanation by using speed. Try and divorce speed from the simple speedometer exmple you used (which is quite correct) and substitute velocity in my examples. Alternatively, I could go back and edit the points to clarify this, your choice. Does that get you past step 2?
Mind you, I am no physicist, or aerodynamicist, and I have never been mistaken for one! Heedm may be able to offer a better more technical explanation of speed/direction/velocity.
Good luck
Edited for (some) of the spelling mistakes!
[ 18 December 2001: Message edited by: helmet fire ]