From Newsgroup: sci.physics.research

Newton's third law says that if body A exerts a force on body B, body B
reacts with an equal and opposite force against body A.

Newton, speaking of inertia, says: "A body exerts this force [inertia]
only, when another force, impressed upon it, endeavors to change its condition".

It seems that he is talking about exactly the same forces.

If not, what differences are there between the two?

Luigi Fortunati
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From Newsgroup: sci.physics.research

On 2025-04-05 20:31:24 +0000, Luigi Fortunati said:

Newton's third law says that if body A exerts a force on body B, body B reacts with an equal and opposite force against body A.

Newton, speaking of inertia, says: "A body exerts this force [inertia]
only, when another force, impressed upon it, endeavors to change its condition".

It seems that he is talking about exactly the same forces.

If not, what differences are there between the two?

Newton's language and the language of Motte's translation are archaic.
Current language is cleared but it was developed much later.

Inertia is not a force. It is a phenomenon. Force is a number or vector
that quantifies an interaction.
--
Mikko

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Mikko il 06/04/2025 13:50:11 ha scritto:

Newton's language and the language of Motte's translation are archaic. Current language is cleared but it was developed much later.

Inertia is not a force. It is a phenomenon. Force is a number or vector
that quantifies an interaction.

Newton, with his archaic language, when he wrote "force" meant force
and, explaining inertia, he repeats it nine times (I have highlighted
them below):
"The vis insita, or innate *force* of matter, is a power of resisting,
by which every body, as much as it lies, endeavors to persevere in its
present state, whether it be of rest, or of moving uniformly forward in
a right line. This *force* is proportional to the body whose *force* it
is; and differs nothing from the inactivity of the mass, but in our
manner of conceiving it. A body, from the inactivity of matter, is not
without difficulty put out of its state of rest or motion. Upon which
account, this vis insita, may, by a most significant name, be called
vis inertiae, or *force* of inactivity. But a body exerts this *force*
only, when another *force*, impressed upon it, endeavors to change its condition; and the exercise of this *force* may be considered both as resistance and impulse; it is resistance, in so far as the body, for maintaining its present state, withstands the *force* impressed; it is impulse, in so far as the body, by not easily giving way to the
impressed *force* of another, endeavors, to change the state of that
another. Resistance is usually ascribed to bodies at rest, and impulse
to those in motion; but motion and rest, as commonly conceived, are
only relatively distinguished; nor are these bodies always truly at
rest, which commonly are taken to be so".

Was Newton wrong to talk about force? Was he wrong to say that inertia
is a force?

First of all, he doesn't say that inertia is *always* a force but it is "*only* when another *force*, impressed on a body, endeavors to change
its condition".

I point out that this is the exact definition of the third law and he
explains it even better in the following when he writes that: inertia
"is resistance, in so far as the body, for maintaining its present
state, withstands the *force* impressed; it is impulse, in so far as
the body, by not easily giving way to the impressed *force* of another, endeavors, to change the state of that another".

Here there is the inertia of both bodies that act and react
reciprocally.

So, the inertia of the two bodies A and B that are approaching is NOT
force but becomes force (as Newton says) *only* when body A tries to
change the condition of body B and body B tries to change the condition
of body A!

That is, inertia becomes force *only* when the two bodies come into
contact and not before or after.

Only during.

Luigi Fortunati
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From Newsgroup: sci.physics.research

On 2025-04-07 12:26:37 +0000, Luigi Fortunati said:

Mikko il 06/04/2025 13:50:11 ha scritto:

Newton's language and the language of Motte's translation are archaic.
Current language is cleared but it was developed much later.

Inertia is not a force. It is a phenomenon. Force is a number or vector
that quantifies an interaction.

Newton, with his archaic language, when he wrote "force" meant force

That does not make sense. In the midern language the word "force" has
a wide variety of different meanings. It had most of these meanings
already in 1729 when Motte translated Principia in English. However,
one important meaning is newer: a quantity in physics. As Newton did
not define the term before its use in the definition of "vis insita"
it must be intepreted according to its meaning in ordinary English of
the year 1729, or one must interprete the original Latin text according
to the common Latin meanings of the year 1726 when the 3rd edition of
Principia was published.
--
Mikko
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From Newsgroup: sci.physics.research

Mikko il 08/04/2025 10:27:48 ha scritto:

Newton, with his archaic language, when he wrote "force" meant force

That does not make sense. In the midern language the word "force" has
a wide variety of different meanings. It had most of these meanings
already in 1729 when Motte translated Principia in English. However,
one important meaning is newer: a quantity in physics. As Newton did
not define the term before its use in the definition of "vis insita"
it must be intepreted according to its meaning in ordinary English of
the year 1729, or one must interprete the original Latin text according
to the common Latin meanings of the year 1726 when the 3rd edition of Principia was published.

You are right that in modern language we have complicated the concept
of "force" to give it a wide variety of different meanings, which did
not exist in Newton's time.

But in the collision language of any year and any century it has a
single and unequivocal meaning: force is the push that body A exerts on
body B and, also, that which body B exerts on body A.

And why do bodies A and B, when they collide, push each other?

They do so exclusively because of their contrasting inertias: if the
inertia of body A did not wanted to go to the right while the inertia
of body B wants to go to the left, there would be no action and
reaction forces.

In collisions, the opposing inertias of the two bodies are the cause
and the action and reaction forces of the third law are the effects.

In Newton's time and also in our time.

Luigi Fortunati
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