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

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.

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
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.


(For some reason, your post shows up in google groups but is not
accessible. So, I've copied it from elsewhere and replied to my post.)




Jeff

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