From: fortunati.luigi@gmail.com
On Tue, 30 Sep 2025 12:12:28 PDT, "Jonathan Thornburg [remove -color
to reply]" wrote:
>In article ,
>Luigi Fortunati asks
>> Up to this point, I've been talking about the forces F1 and F2 between
>> the car and the pin, and between the towbar and the pin.
>>
>> And not about the action and reaction between car 1 and the towbar .
>>
>> So I ask: is the force F1 of car 1 on the pin equal to the action of
>> car 1 on the towbar ?
>
>There is no direct "action of car 1 on the towbar", because car 1 does
>not apply any forces to the towbar. Rather, car 1 applies a force (F1)
>to the pin, and the pin applies a force (let's call it F3) to the towbar.
>
>If you're asking about the *indirect* action (force) applied to the towbar
>by virtue of car 1 applying a force to the pin and the pin applying a force
>to the towbar, i.e., you're asking whether F1 and F3 are equal, see below.
>
>
>> And is the force F2 of the towbar on the pin equal to the reaction of
>> the towbar on car 1?
>
>There is no direct "reaction of the towbar on car 1", because the towbar
>does not apply any forces to car 1. Rather, the towbar applies a force
>(F2) to the pin, and the pin applies a force (let's call it F4) to car 1.
>
>If you're asking about the *indirect* reaction (force) applied to car 1
>by virtue of the towbar applying a force to the pin and the pin applying
>a force to the towbar, i.e., you're asking whether F2 and F4 are equal,
>see below.
>
>
>To summarize what we know so far, we have the following forces acting:
>
>towbar: force F3 to the right, applied by the pin
>
>pin: force F2 to the left, applied by the towbar
> force F1 to the right, applied by car 1
>
>car 1: force F4 to the left, applied by the pin
>
>and we know by Newton's 2nd law (applied to the pin) that
>
> m_pin a = F1 + F2 , (1a)
>i.e.,
> F2 = -F1 + m_pin a and F1 = -F2 + m_pin a , (1b)
>
>where /a/ is the common acccleration of all three (rigid) bodies.
>(Hence if /a = 0/ then F2 = -F1, but if /a/ is nonzero then F2 is not
>equal to -F1.)
>
>
>You've said in the past that you don't think Newton's 3rd law is always
>valid, but if we accept it for the moment, it tells us the answer to your
>questions.
>
>In particular, if we apply Newton's 3rd law to the towbar/pin interface,
>it says that F3 = -F2, which by (1b) means F3 = F1 - m_pin a. So the
>answer to your first question is that if /a = 0/ then F3 = F1, but if
>/a/ is nonzero then F3 is not equal to F1.
If we start from Newton's third law, we find that Newton's third law
is correct.
>And if we apply Newton's 3rd law to the pin/car 1 interface, it says
>that F4 = -F1, which by (1b) means F4 = F2 - m_pin a. So the answer
>to your second question is that if /a = 0/ then F4 = F2, but if /a/
>is nonzero then F4 is not equal to F2.
>
>
>If we *don't* want to use Newton's 3rd law, I suspect we can still work
>out the answers to your questions by using Newton's *2nd* law repeatedly
>on different combinations of the 3 bodies, in the manner of my post to
>this newsgroup a few months ago,
So far it's just your suspicion.
> Newsgroups: sci.physics.research
> Subject: derivation of Newton's 3rd law from 2nd law (was: Re: The
experiment)
> Date: 17 Jun 2025 21:03:56 +0100 (BST)
> Message-ID:
>
>I might do this in a future posting.
Ok, I will wait for this future post of yours.
>ciao,
Ciao.
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