What is the difference between “stiff” and “rigid”?

Could an object be stiff but not rigid or vice versa?

When is each one used?

And what is the opposite of each of them?

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in principle each word in the language should have a slight difference in its meaning than the other synonyms ! otherwise the word itself will not exist ! – ABC-biophi Oct 26 '12 at 4:51
This question is liable to be closed for lack of research. Please see the FAQ. If improved, it can be reopened. – MετάEd Oct 26 '12 at 4:54
alyazan, I disagree with your assessment that each word must have its own nuance of meaning, otherwise, it wouldn't exist. (Two words could have different roots, for example.) – J.R. Oct 26 '12 at 9:01
meta issue: Is this kind of down voting motivated or something? – Kris Oct 26 '12 at 14:44
@Kris ~ the same thought occurred to me... – Roaring Fish Oct 27 '12 at 5:57

In engineering mechanics a rigid body will undergo no deformation whatsoever under any amount of force.

In contrast, stiffness is a measure of how much force it takes to deform a body by a given amount.

In general terms something is said to be stiff when it has a high stiffness coefficient, i.e., it takes a lot of force to deform it; but notice that it does deform.

As the stiffness goes to infinity the body becomes rigid, i.e., no amount of force will be able to deform it.

In practice, nothing is truly rigid, but many times assuming a rigid body simplifies the math needed to analyze a system and when the stiffness of the body is high enough the effects of the assumption are negligible.

Therefore in practice, I reserve the use of the word rigid for cases where there is no possibility for bending (whether figuratively, or literally) and use the word stiff when I believe bending is possible, albeit only with a large amount of force.

As for opposites:

opposite of rigid - deformable : able to be deformed

opposite of stiff - flaccid : not firm or stiff

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• Rigid; not flexible or pliant.

• Of the body, limbs, joints, muscles, etc.: lacking suppleness, unable to move without pain (esp. owing to age, cold, injury, disease, exhaustion, etc.).

Compare with:

I. Stiff, firm, unbending.

1. Of a person or part of the body.

• Stiff, unbending; tense. Also, of a posture or physical response: characterized by stiffness or muscular tension.

The simplified and generalised answer is that, in modern usage, if it usually moves but now doesn't, it is stiff. If it doesn't usually bend, it is rigid.

To answer your questions, yes - something can be stiff but not rigid. A door-hinge for example, when you have to heave on the door to get it open, is stiff but not rigid. COCA gives one lonely hit for 'stiff hinge' and none at all for 'rigid hinge'. Interestingly (given that COCA is American English), Ngram disagrees, and for American English says rigid hinge is more used, but for British English it is a stiff hinge. This reflects how much of a grey area this is...

The reverse is true too. A pole would be described as rigid if it doesn't bend, but not usually described as stiff. COCA gives two hits for rigid pole, and one for stiff pole. Ngram agrees with this one., at least for the past 60 years.

The opposite of stiff is mobile, and the opposite of rigid is flexible.

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I like the hinge example. (As an American, though I'd be more likely to use "stiff" to apply to a door or its hinge.) As for your conclusions about which is more prevalent, it appears that those numbers are too low to draw any meaningful conclusion. – J.R. Oct 26 '12 at 13:43

I suggest the following antonyms:

• rigid - - - yielding (responds to an applied force by changing)
• stiff - - - pliable (readily changes shape under a small applied force)
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It's unfair to the average English learner but the answer is not that simple.

Stated simply, the words are not always synonymous. Different fields of learning have defined the words differently to distinguish between different qualities.

Depending on the domain and context, appropriate meaning will apply.

Free from all context, in general English writing, the words have been widely used interchangeably to imply a resistance to changes (esp., in shape).

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