Control engineers do it blindfolded

[From Rick Marken (960712.1330)]

Hans Blom (960712) --

Yes, these examples are clear. Now please come up with a plausible
example that starts with: "If gravity is ..., then ...".

Bruce Abbott (960712.1235 EST)

If gravity is exerting a downward force against my arm (disturbance) while I
attempt to hold it in a horizontal position (reference), then I must exert a
constant upward counterforce (output) to hold it there.

Remove that disturbance (put me into orbit or into a tank of water in which
my arm has neutral bouyancy) and I can relax (zero output).

How'd I do?

I think you did GREAT, Bruce! But, then, I'm the dufus who thought people
couldn't thread needles blindfolded.

Look out, LA, I'm firing up my model-based output generator and driving
home blindfolded tonight - - just like the rest of you!

Best

Rick