zut:
It seems to me experiments here are extremely loosely stated. You erect a weightless flag pole, and then the next thing you’re doing is climbing a ladder I hadn’t heard about, and I just have to assume the ladder is not on the flagpole (or perched on a dry cloud or a two-headed seagull).
I’m not sure how weightless things act when compressed or stretched. Is the flagpole made of photons, neutrinos or what? And leave me outta this experiment. I’ve never boned up on how to sit on even a normal flagpole, let alone a weightless, sawed-off thingy.
But I don’t understand why you’re discussing all this subtraction of compressive and tensile forces; I well understand them. I just didn’t dig what was going on with why we should worry about whether the forces, as pressures, were measured with respect to the local air pressure or wrt the zero absolute pressure. I just thought we were interested in how they subtract out, exactly as you discuss here. Jdnewmil commented on what was with the absolute measurement, but I don’t really understand it. Go ahead, kick the sawed off flagpole. . .out of “shear” joy.
I don’t know whether liquid bodies can be in tension, other than at their surfaces, or not; but I certainly don’t think gaseous materials can be in tension – at least, when you wish to think of them as perfect gases. Even when perfect gases have zero absolute pressure, it seems to me they expand infinitely. How could you stretch them with added tension? But whether or not liquids can be in tension, I certainly think they tend to hold themselves together by intermolecular forces in zero-tension, zero-compression situations. I certainly appreciate ZenBeam’s flagging somebody’s comment about the air’s being under “tension”.
R a y (to stretch a point, under tension, but not paying much at-)