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Re: [Phys-L] feeler-dealer, third law, et cetera



Quick reply.
"If this means net force isnt a force..."
That seems to be looking for conflict. Net force isnt *A* force it is a sum of forces. An account balance is neither a debt nor a deposit. It is a sum of these things (not an ideal analogy, but time is limited now).
Going back to dealer/feeler notation...what is the dealer for the net force?
Maybe not the most common way to state n3l, but the way it is discussed in some circles, forces are referred to as interactions. Interactions come in pairs...
Gotta go. More work to do.


.:. Sent from a touchscreen .:.
Paul Lulai



-------- Original message --------
From: John Denker
Date:12/11/2013 11:15 PM (GMT-06:00)
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] feeler-dealer, third law, et cetera

On 12/11/2013 08:48 PM, Paul Lulai wrote:
For clarity...
the example of a block experiencing a force of 20 N left and another force of 5N right for a net of 15N does solve the riddle posed.
However, it is not a N3L violation. The 15N is the sum of the forces. Theere is an object experiencing 20N right and another experiencing 20N left. Likewise there is an object experiencing 5N right and another experiencing 5N left.
Lest someone think this violates n3l.

Well, I do think the example is in serious conflict with the
way the third law is usually stated.

Let's take it step by step:

1) We agree that 20 N left is a force.

2) We agree that 5 N right is a force.

3) Question: Is 15 N left also a force?

4) Question: Does the third law apply to /all/ forces?

5) Question: Is this a violation? If not, why not?

==================

Here are several attempts to answer these questions:

First attempt:

3) Yes. Force is a vector. In accordance with the axioms that
define a vector space, the sum of two vectors is a vector.

More formally: Any vector space is closed under addition.

4) Tentative yes. As usually stated, the third law applies to
all vectors.

5) Yes, this is a violation, because there is no discernible
counterpart to the 15N force.

===

Second attempt: Let's back up two steps and try to fix things:

3) We could say that the 15N "force" is not really a force, so it
is not covered by the third law.

IMHO this is a disaster. It means force is not a vector. It
leaves us unable to apply formulas like F=ma to the net force.
Please let's not go there.

===

Third attempt: Let's back up only one step, and try to fix things
a different way:

3) Yes, force is a vector.

4) No, the third law only works when applied to a /subset/ of the
vector space, a subset of all forces. We define a concept of
"simple force" such that the simple forces come in pairs, equal
and opposite. Other forces not so much.

5a) The /restated/ third law works just fine, applicable to /simple/
forces only.

5b) The usual unrestricted third law is not reliable; it gets
violated day in and day out.

5c) If we reformulate the third law in terms of conversation of
momentum, i.e. conservative flow across the boundary between
regions, it works just fine, without restrictions.

http://www.av8n.com/physics/force-intro.htm


========================
In the context of
http://www.av8n.com/physics/force-intro.htm#fig-feynman-disk-field-momentum

On 12/11/2013 10:01 PM, Bernard Cleyet wrote:

This is a betatron?

It's similar in spirit. The relevant force law is the same.
However.....

-- Minor difference: All the betatrons I've seen used an
increasing magnetic field, whereas Feynman's widget uses
a decreasing magnetic field.

-- More important difference: The usual betatron weighs
many tons and is securely anchored to the floor, so there
are lots of ways that momentum could come and go and you'd
never notice. In contrast, Feynman's widget is self-contained.
Also, it is free to pivot, so there cannot be any Z angular
momentum coming in from outside.

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