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To my understanding, Newton’s third law states that for every action there is an equal and opposite reaction. Therefor if I punch the non-Newtonian fluid harder, there will be a harder reaction force stopping my hand. So why is the fluid called non-Newtonian?

I know my logic is flawed, but due to my inexperience, I don’t know where.

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    $\begingroup$ The answer is right there in the first sentence of the Wikipedia article you linked to :) Newton is famous for more than Newton's laws of motion, so "Newtonian" doesn't necessarily refer to that. $\endgroup$
    – Filip Milovanović
    Commented Mar 23, 2022 at 0:29
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    $\begingroup$ Non-Neutonian does not refer to Newton's 3rd law. It refers to Newton's Fluid law/equation which describes fluids and thus such fluids which obey Newton's Fluid law are called Newtonian Fluids and those that don't are not Newtonian $\endgroup$
    – slebetman
    Commented Mar 23, 2022 at 4:18
  • $\begingroup$ This is denying the antecedent: if it violated the third law, that would be a non-Newtonian aspect of it, it doesn't violate the third law, therefore it isn't non-Newtonian. $\endgroup$
    – Acccumulation
    Commented Mar 23, 2022 at 6:29

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Newtonian fluids are named after Isaac Newton, who first used the differential equation to postulate the relation between the shear strain rate and shear stress for such fluids.
For Newtonian fluids there is a direct proportionality between the two quantities and so for fluids which that is not so are called non-Newtonian.

So just like the three laws of motion, law of gravitation, the rings due to interference of light (Newton first investigated them quantitatively), law of cooling, etc, Newton was (amongst) the first to investigate phenomena quantitatively and thus he is heralded for doing this by having his name used when describing various physical effects.

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The non-Newtonian denotion is not about Newton's 3rd law. It is just a referral to the, shall we say, "expected behaviour". And such "expected behaviour" is that when you push harder (shear stress), the liquid moves out of the way faster (shear velocity) proportionally or linearly.

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By DirectEON (talk) 08:46, 28 March 2008 (UTC) - self-made, CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=16635877

Water is (basically) Newtonian. But substances like quick sand, paint or toothpaste are obviously not, since they for instance become unproportionally harder under load (so a quick jerk in your leg will only make it more stuck) or become less viscous (more "flowing") under load (that is why you stir your paint bucket and squeeze your toothpaste tube) and so on.

I believe the term "Newtonian fluid" has been coined due to Newton being one of the first to try to describe liquid behaviour under stresses via differential equations with originally linear solutions.

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