Re: The experiment

Mikko il 01/06/2025 20:06:05 ha scritto:

On 2025-06-01 08:10:52 +0000, Luigi Fortunati said:
>

Is the equality between action and reaction based exclusively on the
third law formulated by Newton and considered so obvious that it never
needed any experiment to confirm it, or has some experiment actually
been carried out?

>
Newton based the low on experiments performed before he wrote the
Principia. Later experiments have not found any deviation from the
law.

What are these experiments on which Newton based his third law and what
are the subsequent ones?

The equality of action and reaction can be inferred from the law of
conservation of momentum, which is also confirmed by all experiment.

I aborted 27 answers before giving birth to this one, the substance was
always the same but one was too verbose, the other with too many
numbers and too many equations, the other too complicated and so on.

I also wanted to do this one again from scratch but I said "enough
now!".

Well then, it is absolutely true that the conservation of momentum is
confirmed by all experiments but it is not at all true that from it one
can deduce the equality between action and reaction.

The law of conservation concerns the two bodies together without any
distinction between the momentum of body A and that of body B, while
the third law concerns precisely the relationship between the single
body A and the single body B.

The law of conservation only tells us how much overall momentum the
bodies have (together) before and after, and does not tell us whether
body A has transmitted to body B more (or less) momentum than it has
received from body B.

If body A unloads more momentum onto body B than it receives, it is
true that the sum of the 2 momentums remains unchanged but the third
law is falsified because the action is not equal to the reaction (I had
also added a mathematical demonstration here but it weighed down my
answer and I eliminated it but I am also ready to deal with numbers).

The conclusion is that the conservation of momentum is not at all
sufficient to demonstrate the validity of Newton's third law.

So, I repeat the question: have there been experiments confirming the
third law or have we always blindly trusted Newton accepting the third
law without any experimental verification?

Luigi Fortunati

[[Mod. note --
There are lots of experiments which support Newton's *2nd* law.
Given Newton's 2nd law, there's a gedanken-experiment which lets us
derive (or at least strongly argue for) Newton's *3rd* law.  Briefly,
the gedanken-experiment has 3 bodies touching each other
   A B C
with an external force pushing right on A (which then pushes right
on B, which then pushes right on C).  We apply Newton's 2nd law to B,
and then consider the limiting case where B becomes very thin in the
horizonal direction (e.g., maybe B is a sheet of aluminum foil oriented
vertically) and B has very small mass.

This weekend I'll try to post a more detailed analysis of this
gedanken-experiment and what we can infer from it about Newton's 3rd
law.
-- jt]]