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stiff and rigid what is the difference?
5

stiff and rigid what is the difference?

stiff and rigid what is the difference?

(OP)
Question was asked in Thread821-106364
I thought I knew but like another in the thread am no longer so sure and felt this is the best forum to ask the question:
Is stiff interchangeable with rigid and vice-versa?
If something is stiff is it the same as something that is rigid? If something else is stiffer or less stiff and there are degrees of stiffness, likewise can something else can have more or less rigidity and have degrees of rigidity that likewise can be measured?

RE: stiff and rigid what is the difference?

it depends on who you talk to.  Stiffness is the ability of something to resist deflection.  Rigidity is essentially the same concept, but is often used differently in conversation.  If someone refers to something as "rigidly mounted," he probably means that the mounting system is very stiff in comparison to a flexible mounting system like rubber feet.  If someone is speaking hypothetically and refers to a structure being "completely rigid," he no doubt intends to confer that deflection of the structure is to be neglected.  In my opinion, for the purposes of general conversation, "stiff" indicates that something deflects appreciably but grudgingly, and "rigid" means it is intended not to deflect significantly under the expected range of loads.

RE: stiff and rigid what is the difference?

2

In general, they appear to be interchangeable. However, one source tells that mechanical engineers refer to rigidity as the quality or state of resisting change in form, while defining stiffness as the ratio of a steady force acting on a deformable elastic medium to the resulting displacement. There are stiffness coefficients, constants, and matrices.

Other disciplines have different definitions for stiffness and rigidity.

Physics, for example, refers to "rigid" bodies as objects whose constituent particles remain in fixed orientations relative to each other like planets, baseballs and cars.

In conclusion, ivymike is correct, it appears there is no rule, and that we should use them as befitting the particular branch of engineering or science which we are involved with.

RE: stiff and rigid what is the difference?

(OP)
Thank you ivymike & 25362 for your explanations. Both worth a star.
So, then, does rigid infer that for practical purpose movement is not possible and stiff infer that movement is possible, relative to a given load?

RE: stiff and rigid what is the difference?

Stiffness and rigidity could also relate to relative motion. A stiff object can move/translate as an object but a rigid obejct could be inferred to be fixed in space, regardless of whether their shape distorts.

corus

RE: stiff and rigid what is the difference?

No way.  That would eliminate the whole concept of "rigid body motion", one of the basics of freshman physics.

Rigid bodies don't distort.  Like 25362 says, "constituent particles remain in fixed orientations relative to each other".  But the rigid body as a whole can move relative to some other object or point in space.

I've only heard "stiff" used to mean "relatively stiff", never absolutely rigid.

In conversation, as opposed to engineering or physics, the terms are pretty interchangeable.

Hg

RE: stiff and rigid what is the difference?

Below are the definitions of both, taken straight out of on-line dictionary.(www.dictionary.com)

For those who don't have time to read that verbal diarhorrea, don't bother. The both words seem to have the same (or basically the same) meaning in technical language. Use one of your engineering tools (hopefully): common sense and apply your wording to fit the context. To me "Stiff" seems somehow less formal than "Rigid".

Otherwise refrain from stiff drinks during pondering that problem and don't be to rigid in your judgment.


Stiffness:

Difficult to bend; rigid.
Not moving or operating easily or freely; resistant: "a stiff hinge."
Lacking ease or comfort of movement; not limber: a stiff neck.
Drawn tightly; taut.
Rigidly formal.
Lacking ease or grace.
Not liquid, loose, or fluid; thick: stiff dough.
Firm, as in purpose; resolute.
Having a strong, swift, steady force or movement: a stiff current; a stiff breeze.
Potent or strong: a stiff drink.
Difficult, laborious, or arduous: a stiff hike; a stiff examination.
Difficult to comprehend or accept; harsh or severe: a stiff penalty.
Excessively high: a stiff price.
Nautical. Not heeling over much in spite of great wind or the press of the sail.


Rigidness:
Not flexible or pliant; stiff.
Not moving; fixed.
Marked by a lack of flexibility; rigorous and exacting: “We have watered down a rigid training... until we now have an educational diet in many of our public high schools that nourishes neither the classes nor the masses” (Agnes Meyer).
Scrupulously maintained or performed: rigid discipline. See Synonyms at stiff.

Putting Human Factor Back in Engineering

RE: stiff and rigid what is the difference?

The stance is usually stiff but the stand is always rigid.

Hope this has helped immensely.

RE: stiff and rigid what is the difference?

(OP)
I like  this definition from the University of Winnipeg.

"............Definition: Rigid Body

A rigid body is one that does not deform during its motion. The distance between any two points in the rigid body remains fixed............"
HGTX, a long overdue star.





  

RE: stiff and rigid what is the difference?

A stiff is also slang for a dead body

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