To Robert Carlson and and all who think like him, Dear Robert:
I must disagree that F = m a is just a definition. Of course, here
we may slip into deeper philosophy: is velocity just a definition?
Acceleration? The fact is that forces DO exist even in the absence
of motion (if you wish to be convinced, just stand and hold a 20 kg
box for 10 minutes). The whole part of mechanics called "statics" deals
with forces where there is no motion, and with the laws of statics
people built skyscrapers, bridges and so on. Since no motion implies
acceleration = 0, there should be no forces there and it is hard to
imagine a whole branch of physics dealing with nothing.
Regards Emilio