Re: Here's one for Bob (hope it makes your head spin)

On 09/07/07 18:27, Doug Phillips wrote:
On Sep 7, 5:23 pm, Ron Johnson <ron.l.john...@xxxxxxx> wrote:
On 09/07/07 16:27, Doug Phillips wrote:



Ron Johnson wrote:
On 09/07/07 10:08, Doug Phillips wrote:
On Sep 7, 7:40 am, Ron Johnson <ron.l.john...@xxxxxxx> wrote:
Guns use chemically unstable materials to "[produce] a sudden
expansion of the material usually accompanied by the production of
heat and large changes in pressure (and typically also a flash
and/or loud noise) upon initiation; this is called the explosion.]
The gas expanding in the confined area of the barrel "blows" the
projectile out the barrel like a breeze blows a leaf, or a person
blows a feather with his breath.
OTOH, rockets "[obtain] thrust by the reaction to the ejection of
fast moving fluid from within a rocket engine."
However... the gun's recoil is an expression of Newton's 3rd.
Newton's third law: "For every action there is an equal an opposite
reaction."
You have described actions -- chemicals exploding, gas expanding,
breeze and breath blowing -- and named the reactions to those actions.
Where is the law not applicable in any of your examples?
How is "breeze pushing a leaf" an opposite reaction?
<http://en.wikipedia.org/wiki/Newton's_laws_of_motion>
A more precise statement of the third law can be found there:
"LAW III: To every action there is always opposed an equal reaction:
or the mutual actions of two bodies upon each other are always equal,
and directed to contrary parts. - Whatever draws or presses another is
as much drawn or pressed by that other. If you press a stone with your
finger, the finger is also pressed by the stone. If a horse draws a
stone tied to a rope, the horse (if I may so say) will be equally
drawn back towards the stone: for the distended rope, by the same
endeavour to relax or unbend itself, will draw the horse as much
towards the stone, as it does the stone towards the horse, and will
obstruct the progress of the one as much as it advances that of the
other. If a body impinge upon another, and by its force change the
motion of the other, that body also (because of the equality of the
mutual pressure) will undergo an equal change, in its own motion,
toward the contrary part. The changes made by these actions are equal,
not in the velocities but in the motions of the bodies; that is to
say, if the bodies are not hindered by any other impediments. For,
because the motions are equally changed, the changes of the velocities
made toward contrary parts are reciprocally proportional to the
bodies. This law takes place also in attractions, as will be proved in
the next scholium."
Hope that helps.
It does. The ground pushes up against me as I walk, the table
pushes up against the book, etc.

But it does not (yet, to me) explain the "excess" force from the
expanding gas which accelerates the projectile down the barrel.

Another example: it is N3 that keeps a stationary rubber ball *on*
the table, but it is the "excess" force from gravity, a throwing
arm, what the ball and table are made of, etc, which causes it to
bounce back off the table.

What am I misunderstanding?


All three of Newton's law apply to all (classical) motion: inertia,
acceleration and reciprocal actions. (Note that the 3rd law applies
even if actual "motion" doesn't occur because the opposing forces
offset each other.) All three laws apply to what happens in a gun
chamber, just as they do to you or me typing at a keyboard or walking
down the street. You're right that other laws come into effect, but
they don't invalidate Newton's.

I'm not saying that N3 is *invalidated*, just not used by the bullet.

In a gun chamber, the force of the explosion is exerted against the
"containment" and the containment resists. If the resistance isn't
sufficient, the force will continue outward until it has been absorbed
(accumulated resistance) to offset it.

Inertia and acceleration.

I understand that completely.

The bullet, having an "escape" will not (should not) be able to resist
as much and some of the force will be transfered into motion -- but
plug the barrel and the gun will likely explode if the force is too
great to contain.

Because of inertia and acceleration.

"The changes made by these actions are equal,
not in the velocities but in the motions of
the bodies; that is to say, if the bodies are
not hindered by any other impediments."
If the gun were not allowed to recoil (the direction of which is a
reaction of the bullet's motion)

And that of the expanding gas.

the forces would still interact
within the entire chamber.

But the gun *must* recoil. It's N3. A force meter on a strong,
stiff backstop would measure it, even if the gun didn't move.

I'm sure I've oversimplified it, and I hope I haven't misused any
terms that further add to confusion. If I have, I'm pretty sure
someone will jump in to chastise me and will provide you with a better
explanation.

Sorry. Since the bullet travels I still don't understand how N3
makes the *bullet* fly out the barrel.

Hunger and thirst now call me elsewhere.



--
Ron Johnson, Jr.
Jefferson LA USA

Give a man a fish, and he eats for a day.
Hit him with a fish, and he goes away for good!
.

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