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From: Luigi Fortunati <fortunati.luigi@gmail.com>
Newsgroups: sci.physics.research
Subject: Re: The rope
Date: Fri, 23 May 2025 14:37:54 PDT
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Luigi Fortunati il 21/05/2025 05:03:11 ha scritto:
> In the diagram https://www.geogebra.org/classic/pnbsvfuk there is the 
> hand holding end A of the 2 kg rope and there is end B of the rope 
> holding the 15 kg bucket full of water.
>
> The hand exerts the blue force +17 (upward) on the rope, and the rope 
> reacts with the red force -17 (downward).
>
> And the rope exerts the blue force +15 (upward) on the bucket, and the 
> bucket reacts with the red force -15 (downward).
>
> So, contrary to what Newton says, the rope does NOT exert the same 
> force on the hand (-17) and on the bucket (+15)!
> [[Mod. note -- 
> Newton's 3rd law applies separately at each location where forces are
> applied:
> * At the top of the top, Newton's 3rd law says the hand force on the
>   rope (17 upward) is equal in magnitude and opposite in direction from
>   the rope force on the hand (17 downard).
> * At the bottom of the top, Newton's 3rd law says the hand force on the
>   bucket (15 upward) is equal in magnitude and opposite in direction from
>   the bucket force on the hand (15 downard).

You probably meant here that the forces +15 and -15 at the bottom of 
the top are between the rope and the bucket and not between the hand 
and the bucket.

In any case, this is all your interpretation and not that of Newton who 
writes: "[the rope] will impede the progress of the stone as much as it 
will promote the progress of the horse".

So, for Newton the rope *always* pulls equally at both ends and never 
with force 17 on one end and 15 on the other.

> But, the top of the rope and the bottom of the rope are different
> locations (with different forces applied), so Newton's 3rd law does
> not say anything about how the top-of-the-rope forces and the
> bottom-of-the-rope forces relate to each other.

And in the tug of war, are the forces on the side of team A (which are 
at different points) related to those on the side of team B?

Luigi Fortunati