# Newton's third law pair

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21 hours ago, swansont said:

I said that no reaction forces are depicted in the diagram (no forces exerted by the derrick are shown), and you argued with me about it!

The diagram shows two forces, and states that the normal force from the rock counters gravity. "Counters" is used to show the forces add to zero, and give a zero net force. It does not have any force arrows depicting forces exerted by the derrick.

It's obviously implied. There are no force arrows depicting forces exerted by the book. Yet somehow you were able to made this claim:

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A few posts later, I said "The reaction force to the normal force exerted by the table on the book would be the normal force exerted by the book on the table."

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The normal force exerted by the derrick does not "counter" the normal force exerted by the rock - they are not acting on the same thing, so they cannot be combined to calculate a net force.

Nonsense. The normal forces are third law pair they can't act on the same object.

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Ghideon stated that in the second post in the thread (the first response).

In my second post, I stated that Ghideon was correct. A couple of posts later, studiot agreed.

A few posts later, I said "The reaction force to the normal force exerted by the table on the book would be the normal force exerted by the book on the table."

So aside from us three (and whoever else; I stopped looking at that point), sure, nobody else acknowledged it...

How convient? I kept repeating that argument yet somehow I missed every single reply which confirmed it. That seems suspect just like one of my replies which was butchered even though it was copied and pasted from wordpad with no error.

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Action/reaction is not dependent on equilibrium. I know this because you can have action/reaction forces when a system is not in equilibrium. I gave an example - the earth/moon system. They each exert a gravitational force on each other, equal in magnitude, opposite in direction. That's action/reaction. And yet, no equilibrium

Comparing conditions of objects that are in static equilibrium with objects that are not in equilibrium is pointless. Static equilibrium must contain a suitable third law pair.

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Equilibrium, if it exists, is a matter of the second law. Not the third.

Absolutely wrong. Equilibrium depends on the conditions of all three laws of motion. However, the third law is fundamental to reactions for all static equilibrium.

21 hours ago, J.C.MacSwell said:

Don't feel bad about me though...others have tried much harder to make points you prefer to ignore.

(I quoted you directly when I posted that)

Sure, I just happened to miss that particular reply from you. Meanwhile your other replies to me on the exact argument gave no such claim:

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Because the fact that the forces are equal but opposite are coincidental...(giving us a static case due to the forces balancing)

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That is the definition of a normal force. Sometimes it is a component of force, say for something sitting on an incline plane, the weight would be counterbalanced by the normal force and the force of friction.

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38 minutes ago, Nedcim said:

It's obviously implied. There are no force arrows depicting forces exerted by the book. Yet somehow you were able to made this claim:

How is it "implied"?

This is the derrick example. There is no book

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Really? You're going to claim this without explaining it? AFAIK I have been completely consistent

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Nonsense. The normal forces are third law pair they can't act on the same object.

I give up. This is what I said - that they can't be combined because they don't act on the same object, and you say it's nonsense for the very reason I explained.

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How convient? I kept repeating that argument yet somehow I missed every single reply which confirmed it. That seems suspect just like one of my replies which was butchered even though it was copied and pasted from wordpad with no error.

I assume you are capable of scrolling back through the first page of the thread. Yes, you somehow missed this. Your only post in this span was about how you were still confused about the answer in the OP.

"I still have no clearer understanding why the example is not an an action-reaction pair of Newton's third law. "

And despite this, somehow now you're an expert on Newton's laws...

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39 minutes ago, Nedcim said:

Sure, I just happened to miss that particular reply from you. Meanwhile your other replies to me on the exact argument gave no such claim:

An earlier reply, again directly quoting you...(brick not book at that time)

On 5/25/2020 at 8:21 PM, J.C.MacSwell said:

Because the fact that the forces are equal but opposite are coincidental...(giving us a static case due to the forces balancing)

As already suggested, if the table was accelerating upwards they would not be equal.

If the table was frictionless but not perfectly level the forces would not be opposite.

But as mentioned all these scenarios only involve forces that have equal but opposite counterparts. Every action having a reaction. For the static case the force the table exerts on the object balances the Earth's gravitational pull on the object... but that is not the equal but opposite force of Newton's third law. The equal but opposite gravitational force on the brick is the brick's equal but opposite gravitational force on the Earth...and the equal but opposite force of the table on the brick is the force of the brick on the table.

Have you learned anything?

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45 minutes ago, Nedcim said:

How convient? I kept repeating that argument yet somehow I missed every single reply which confirmed it. That seems suspect just like one of my replies which was butchered even though it was copied and pasted from wordpad with no error

Which part of my initial anser do you find suspect?

In case my follow ups and questions are unclear please post some feedback so I’ll can try to add further explanations.

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51 minutes ago, Nedcim said:

How convient? I kept repeating that argument yet somehow I missed every single reply which confirmed it. That seems suspect just like one of my replies which was butchered even though it was copied and pasted from wordpad with no error.

!

Moderator Note

After six pages, it's clear you aren't arguing in good faith. First you claim not to understand, then you're an expert. Every time you're pressed on a point, you ignore it and jump to another example. This isn't how discussion works if one is concerned with filling a gap in one's knowledge with something trustworthy. You've had ample time to support your arguments rigorously, and you haven't. Don't bring this up again if this is the way you're going to "discuss" science.

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