My dear Gary, how could I forget that in post number1 there is a line
saying "An airplane on a runway sits on a conveyer belt that moves in
the opposite direction at exactly the speed that the airplane is moving
forward..." so in other words, the belt adapts it's speed to the
forward motion of the plane?? Another communication problem I guess.
I'm not surprised though, after 55 freakin' posts this topic is
becoming off limits in my personal opinion! But I do hope that now
everyone agrees that the explanation on that website is correct. It's
just a matter of explaining the question properly with all details and
eliminating possible communication problems, right?

No, this is strictly Galilean relativity (from four centuries ago);
Einstein's relativity has no bearing on this discussion.

I don't agree completely. But your right that Galilean blabla has to do
with this but then saying that it's all about Newton's laws would also
be correct. So does Einstein! May I remind you of one of Einstein's
many mind experiments... The one about a train moving at a certain
speed, an observer inside the train walking to the front of the train
and an observer outside the train, standing still... As I said, it all
depends on what reference you take! To the observer outside the train,
the train may be traveling at 100km/h and the guy inside the train will
be walking at 105km/h! But to the guy inside, the train seems
stationary, while he himself is going at 5km/h and the guy outside to
him appears to be going backwards at 100km/h! It all sounds very
complicated and silly but relativity (and communication problems) is
all this topic is about! Now why don't y'all go and get a nice cup of
coffee and then start another topic.
No hard feelings I hope!