greendoor wrote:Grimer - do you still believe that every force has an equal & opposite reaction?
You're not listening, naughty boy. I keep telling you there is no such thing as force. Force is merely an alias for strain.
Do I believe that every strain has an equal and opposite strain.
No if you measure strains from false stress datums.
Yes if you measure strain as natural strain from the true datums which takes into account the pressure difference between the inside and outside of a material, the pressure differential which holds the material together and gives it its strength and stiffness.
Natural strain entropy's dx/x are equal and opposite in sign but not in direction. The equal and opposite bit is wot Newt recognised instinctively when he gave us "forces are equal and opposite." Unfortunately he didn't see the implications in terms of strain - which is why things acting at near right angles must have been so puzzling when electromagnetism started to be investigated.
Strains are not generally opposite in direction. A longitudinal strain in one phase can be balanced by a lateral strain in the other and normally is when a material is stressed.
The equal and opposite bit can be seem to be an artifact, a fiction even at the macro level of structural analysis.
Suppose you have three struts intersecting at angles of 120 degrees to each other.
Suppose you load one strut.
Where is the opposing force?
There ain't one is there!
The resolution of the two other forces is a mathematical trick and as unreal as the statistician's "average". There is no average man. He is a fiction. And so in the case I have just described is the opposite force.
Forces are manifestly not equal and opposite. Natural strains when measured from absolute stress datums (entropic/ectropic) are opposite, opposite that is in the tensile/compressive sense, not in the directional sense.
Of course if you measure strains from the wrong datum then you will find that the strains are not equal and opposite. If you stress a square inch rod of steel against a square inch rod of concrete then the strain of the steel will be much less than the strain of the concrete cos the steel is much stiffer than the concrete as measured from the false datums of steel and concrete being initially unloaded. To think this is to make the same error as thinking they have the same heat content because they are at the same ambient temperature, the same heat "force" of "zero".
I'd better stop there or you will get mental indigestion if you haven't got it already.
