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

On 09/07/07 21:05, Jeff Campbell wrote:
Ron Johnson 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?

The gun barrel is open at one end (hopefully 8-) ) at the time
propellant in the bullet (shell) is burning. The resistance to
displacement exhibited by the gun barrel walls, stiff and
unmoving 8-) ) versus the lack of resistance (in a vacuum) or
very low value of resistance to displacement exhibited by the
column of air in the gun's barrel when compared to the resistance
of the barrel's walls, forces (no pun intended 8-) ) the bullet
to move toward the barrel's open end as the burning propellant
produces expanding gas pressure. For a short time before the
propellant is totally consumed the resulting gas pressure is
uniformly and equally applied to the barrel walls and the base of
the bullet. The barrels walls do not move (much 8-) ), the bullet
can and does.

Correct. Inertia and acceleration.

/Mythbusters/ is always fun to watch. Firing a 12ga shotgun
submerged in water caused the barrel to explode.

A rocket motor operates in exactly the same fashion. The rocket's
burn chamber is enclosed by walls in all directions except for
the opening that leads to the nozzle. These walls resist
displacement by the gas pressure created by burning the rocket
motor's propellant. The gas pressure opposite the nozzle opening
is what propels the rocket motor (and the stuff attached to it
8-) ) in the same direction as that exerted gas pressure. This
force is why a rocket motor can operate in a

You mean the *opposite* direction as the gas?

vacuum. The exhaust gases leaving the motor via the exhaust
nozzle do not 'push' against the atmosphere to move the rocket,
there being no 'air' in outer space.

Correct. Purely Newton's 3rd Law.

--
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!
.

Relevant Pages

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