From Newsgroup: sci.physics.research

On 2025-07-02 15:50:32 +0000, Luigi Fortunati said:

I watch the animation https://www.geogebra.org/classic/cheqdq5u where,
at the moment of impact, the force of 22 particles pushing to the
right (action) and that of 10 particles pushing to the left (reaction)
hits the wall.

If the particles to the right and particles pushing to the left
don't interact with each other the action and reaction mentioned
above are not action and reaction of the same interaction, Even
if they do there is also mentioned interactions of each particle
with the wall and that interaction has its own action and reaction.

Mathematics tells me that the push of 22 particles is greater than the counter push of 10 particles.

At least if the masses and velocities are the same.

Reasoning tells me that in a real experiment, after the action and the reaction, the wall collapses to the right and not to the left,
demonstrating that the action is greater than the reaction.

Action in one interaction need not be equal to the reaction in another interaction.

What happens to the wall depends on the properties of the wall
in addition to the number and properties of the particles. Which
particles hit first may also have some effect.

Should I blindly believe Newton's third law that tells me that the
action is always equal to the reaction or should I believe my eyes, reasoning, mathematics and the experiment that tell me the opposite?

Newton's third law does not say that action is always the same as
reaction. It only says so about the action and the reaction in the
same interaction. Another way to say the same is that both action
and reaction is the quantity of the interaction.
--
Mikko
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From Newsgroup: sci.physics.research

Mikko <mikko.levanto@iki.fi> wrote or quoted:

Newton's third law does not say that action is always the same as
reaction. It only says so about the action and the reaction in the
same interaction. Another way to say the same is that both action
and reaction is the quantity of the interaction.

Yes, and a wall is no particle but an extended object.

FWIW, an attempt at a small tutorial:

Newton's Third Law: Action and Reaction

Imagine the world is made of tiny "particles." In physics,
we often think of these as points - they have no size, no
shape, just a position. When two particles interact, they push
or pull on each other with forces.

Newton's Third Law is very simple but very powerful. It says:

.------------------------------------------------------.
| For every force that one particle exerts on another, |
| there is an equal and opposite force exerted back. |
'------------------------------------------------------'

Let's say we have two particles, which we'll call particle
"i" and particle "j". The force that particle "j" exerts
on particle "i" is written as:

f_ij

And the force that particle "i" exerts on particle "j" is:

f_ji

Newton's Third Law says:

f_ij = -f_ji

This means that if particle "j" pushes on particle "i" with
a certain force, then particle "i" pushes back on particle "j" with
exactly the same amount of force, but in the opposite direction.

How Forces Affect Motion: Momentum

Now, let's see what happens to the motion of these particles.
In physics, we use a quantity called "momentum" (written as "p")
to describe how much motion a particle has. The momentum
of a particle changes when a force acts on it.

The rate at which the momentum of particle "i" changes is given
by the sum of all the forces from all the other particles:

d
-- p_i = sum of all f_ij
dt (for all j not equal to i)

Here,

- "d/dt" means "the rate of change with respect to time"
- "p_i" is the momentum of particle "i"
- "f_ij" is the force on "i" due to "j"
- The sum is over all other particles "j" (not including "i" itself)

Adding Up All the Particles

Suppose we have lots of particles. Let's add up the rate of change
of momentum for every particle:

d
-- (sum of all p_i) = sum of all f_ij
dt

But here's the trick: For every pair of particles, the forces
they exert on each other are equal and opposite. So, if you
add up all the forces for every pair, they cancel out:

f_ij + f_ji = 0

So, when you add up the right side for all pairs, you get zero!

Conservation of Momentum

This means:

d
-- (sum of all p_i) = 0
dt

Or, in words:

.------------------------------------------------------------------.
| The total momentum of all the particles together does not change |
| over time. |
'------------------------------------------------------------------'

This is called the "conservation of momentum." It's a deep and
important idea in physics: In an isolated system (where nothing
from the outside is pushing or pulling), the total momentum
stays the same, no matter what the particles do to each other.

Key Points to Remember

- Particles are points They have no size or shape, just a
position.

- Action = Reaction Forces between particles always come
in equal and opposite pairs.

- Momentum changes because of forces.

- Total momentum stays the same if the system is isolated.
This is the conservation of momentum.
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