Newton's 2nd law F=ma states that the mass m is acted upon by the
force F and accelerates passively without reacting, also because there
is only one body of mass m and there is no other body on which to
exert the reaction.
However, the 3rd law prohibits an action without a corresponding
reaction.
For this reason, I prepared the animation >https://www.geogebra.org/classic/qn5ucjxp where two bodies of mass
m=1kg are pushed to the right by the force F=+1N with an acceleration
of +1m/s^2, exactly as prescribed by Newton's 2nd law.
And I ask: do the bodies acted upon by the blue force F at point X
react with the opposing red force also directed against point X, or
does the red reaction force not exist?
Luigi Fortunati
[[Mod. note --
...
Fortunately, none of the above affects the answer to the author's question, >which is basically asking if Newton's 3rd law still applies to each body.
The answer to that is yes, it does. That is,
* Body A exerts a force of 1N pushing to the left on whatever body
(not showin in the animation) is applying the rightward force on A.
* Body B exerts a force of 1N pushing to the left on whatever body
(not showin in the animation) is applying the rightward force on B
-- jt]]
Newton's 2nd law F=3Dma states that the mass m is acted upon by the
force F and accelerates passively without reacting, also because there
is only one body of mass m and there is no other body on which to
exert the reaction.
=20
However, the 3rd law prohibits an action without a corresponding
reaction.
I asked about the internal forces F2 and F3 in the animation >https://www.geogebra.org/classic/qn5ucjxp, which act on the wall X of
the same body and not on other bodies not shown in the animation.
Do the water in body A and the *rigid* ice in body B exert their
*internal* forces (F2 and F3) against wall X (unbalanced by the
opposing force against wall Y), or do the unbalanced *internal* forces
F2 and F3 not exist and did I invent them?
Luigi Fortunati.
[[Mod. note --
Ok, now I think I understand what you're asking.
As stated, the problem description is self-contradictory: you've said
that body A and body B each have a total mass of 1kg, but you've also
said that their water and ice masses are also 1 kg each. This doesn't
leave any mass for the tank itself (including wall X)! For simplicity,
let's say that each tank (empty) weighs 100 grams, so that that A's water
and B's ice each weigh 900 grams.
Now to your questions:
Yes, the water in body A exerts an internal force F2 pushing to the left
on wall X. Because the water sloshes in the tank, F2 will be time-dependent. >Analyzing Body A's motion and forces in detail is hard because of the water >sloshing in the tank.
For body B, the answer to your question depends on whether the ice is
stuck to the bottom of B (in which case there's only a very small force
F3), or whether the ice-bottom contact is low-friction (let's idealise
this to say *no* friction). Let's assume the latter.
Apply Newton's 2nd law to wall X: the wall has 2 (horizontal) forces
acing on it, F_external = +1N and F3, so we have
F_net = F_external + F3 = +1N + F3
= m_wall a
= 0.1kg * +1m/s^2
= +0.1N
so we must have F3 = -0.9N. This tells us that there is indeed a force
F3, and tells us what that force is (0.9N pushing left).
[[Mod. note --
Ok, now I think I understand what you're asking.
As stated, the problem description is self-contradictory: you've said
that body A and body B each have a total mass of 1kg, but you've also
said that their water and ice masses are also 1 kg each. This doesn't
leave any mass for the tank itself (including wall X)! For simplicity,
let's say that each tank (empty) weighs 100 grams, so that that A's water
and B's ice each weigh 900 grams.
Now to your questions:
Yes, the water in body A exerts an internal force F2 pushing to the left
on wall X. Because the water sloshes in the tank, F2 will be time-dependent. >Analyzing Body A's motion and forces in detail is hard because of the water >sloshing in the tank.
For body B, the answer to your question depends on whether the ice is
stuck to the bottom of B (in which case there's only a very small force
F3), or whether the ice-bottom contact is low-friction (let's idealise
this to say *no* friction). Let's assume the latter.
Apply Newton's 2nd law to wall X: the wall has 2 (horizontal) forces
acing on it, F_external = +1N and F3, so we have
F_net = F_external + F3 = +1N + F3
= m_wall a
= 0.1kg * +1m/s^2
= +0.1N
so we must have F3 = -0.9N. This tells us that there is indeed a force
F3, and tells us what that force is (0.9N pushing left).
Finally, apply Newton's 2nd law to the ice in body B: The ice has only
one force acting on it,
[I'm assuming that the ice isn't stuck to wall Y,
i.e., that wall Y isn't exerting any horizontal forcw
on the ice.]
a reaction force (let's call it F4) applied by the wall to the ice.
Newton's 2nd law applied to the ice gives
F4 = m_ice a
= 0.9kg * +1m/s^2
= +0.9N
This tells us that wall X pushes right on the ice with a force of 0.9N.
Notice F3 = -F4, i.e., Newton's 3rd law also holds here.
-- jt]]
Based on your own numbers, wall X experiences two forces: the external
force F = +1N directed to the right and the force F3 = -0.9N directed
to the left.
The difference between the two opposing forces (F and F3) is the net
force +0.1N that accelerates wall X without compressing it, while the >remaining opposing forces (+0.9 and -0.9) compress it without
accelerating it.
One force accelerates it and the other two compress it.
How do we detect the net force +0.1N? We measure the acceleration of
wall X.
How do we detect opposing forces on the same body? We measure its >compression.
And now let's move on to the block of ice.
Based on your analysis, the block of ice experiences only one force,
the external force F4 = +0.9N, and nothing else.
If this were indeed the case, the block of ice should accelerate
without experiencing any compression.
And yet, for the entire duration of the acceleration, the block of ice >undergoes a compression that can be measured by highly precise
instruments.
How could the block of ice (or any other material) compress if, as you
say, there is no opposing force?
Luigi Fortunati
[[Mod. note --
Consider an imaginary vertical line dividing the block of ice into a
part /IL/ to the left of the line and a part /IR/ to the right of the
line. The position of the vertical line is such that /IL/ contains a >fraction /f/ of the ice, where /f/ is some as-yet-unspecified number
in the range [0,1], i.e., /IL/ has a mass /f m_ice/ and IR has a mass
/(1-f) m_ice/.
/IR/ is accelerating to the right, so by Newton's 2nd law there must be
a net force /F5/ to the right acting on IR. This force can only come
from /IL/ being compressed and pushing right on /IR/. By Newton's 3rd
law, /IR/ also pushes left on /IL/ with a force /F6 = -F5/
Since uniform acceleration is equivalent to a gravitational field, this >system is very analogous to a rope hanging vertically, where the tension >varies along the rope.
In a following posting I'll work out this analysis in a bit more detail.
How could the block of ice (or any other material) compress if, as you
say, there is no opposing force?
In the vertical-ice system, consider an imaginary horizontal line $hx$
at the position (height) $x$, dividing the ice into a lower part $Ilow$
(of height $x$ and mass $m_Ilow (x/L) m_ice$ and an upper part $Ihigh$
(of height $L-x$ and mass $m_Ihigh = (1-x/L) m_ice$.
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