I offer this nitpick about non-computer issues to the monestary in the interests of academic completeness.
Within an engineering community, stress and strain have an exact meaning (stress => compression, strain => tension), and people who misuse the terms brand themselves as weenies. Or, at least, non-engineers.
Non-engineer, definitely. :) Strain can be thought of as a measure of movement, and stress as a measure of force. Your example sounds to me like you're saying "I weigh 17 inches." - nonsense. So here's a mini-tutorial on stress, strain, and the like, because I know you're dying to learn about solid mechanics today.
Stress is essentially a measure of how much force exists in a unit area of something - a basic expresion is s=p/a (though stress should be written sigma - σ). Stress can be thought of as pressure. Take a pencil, put the point down on your desk, and press on the eraser head - it doesn't hurt. Would it hurt if you had the eraser head on the desk and your palm was pressing against the pencil lead? There is less pencil area for the force of your hand to act on, so the stress is higher. (BTW, if you really want me to, I can explain bending stresses (s=M*c/I) as well.)
Strain is a measure of the change of length of something - expressed as e=change/length (and e should be written epsilon - ε). If I have a spring which is an inch long, and I compress it, it may change length .5 inches. If I take a spring which is 10 inches long, but otherwise identical, and I compress it, I'll get much more length change than .5 inches. Strain is a ratio measure of length change.
A reason that lay-people often use the terms interchangably is that the two are related. Hook's law is an empirical relationship which states that stress and strain are directly related : s=e*E (or σ=ε*E for those browsers which support HTML entities). The constant 'E' is Young's Modulus, also called the Modulus of Elasticity; you can think of it as a measure of how much force it takes to compress a spring, the two concepts are very close.
I don't know what jmcnamara meant when he offered up his example - I'll assume that he knew what stress and strain are. So now you know a little about solid mechanics. | [reply]
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I don't know what jmcnamara meant when he offered up his example - I'll assume that he knew what stress and strain are.
Indeed I do. I have a masters degree in Mechanical Engineer. :-)
That was a very clear explanation of the mechanical concepts. I was referring instead to the vernacular use: "He is under a lot of stress at work, I hope he can take the strain".
And just to out nit-pick you. Hook's law only applies for elastic materials in their elastic range. And even for elastic materials you can have stress without strain and strain without stress. But let's not get into all of that. ;-)
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John.
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I'm a lay-person, so I'd like to add one point. Outside of physics and engineering, strain and stress are used to describe how we are feeling or the effect something is having on us. For example "My kids are stressing me out." or "This job is a strain on my marriage." You could argue that in some small way your definition still works, but people are generally describing feelings which do not correspond neatly to engineering concepts. If I say "All of those trucks are putting a lot of strain on that bridge," I would not be thinking about your definition, but rather the more common definition - the bridge is dealing with something difficult and might break. I might say the same about a relationship "All of those long nights at the office are putting a strain on my marriage." Again, talking about dealing with something difficult (long hours) and something that might break (the marriage).
I agree that way back when, these definitions came from physics/engineering, but now they are in the public domain and so they've taken on a life all their own. The original definitions are just one definition.
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