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

In article <1189426372.388487.37070@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, AEF <spamsink2001@xxxxxxxxx> writes:
On Sep 10, 7:22 am, bri...@xxxxxxxxxxxxxxxxx wrote:
In article <BbkEi.239334$BX3.95...@xxxxxxxxxxxx>, Ron Johnson <ron.l.john...@xxxxxxx> writes:
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.

Nor does it explain the excess force from the expanding gas
in a rocket nozzle or in a gun barrel.

There's a chemical reaction that explains that excess force.

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?

At a guess, you are under the delusion that for every physical situation
that is explainable there is a single explanation that captures
the essential details and that all other explanations are wrong.

You may also be conflating "explanation" and "cause".

Don't forget the recoil of the gun. The gases push the bullet forward
and gun backward.

I'm not sure what is meant here by "excess" force, but maybe this will
help clear it up.

Here's my understanding of "excess" force in context...

The scenario in question is a rubber ball sitting on a table. Or
a bullet sitting stationary in a not-yet-fired cartridge.

There is a "ordinary" force that simply holds the ball or bullet in
place against the force of gravity.

And there is an "excess" force that accelerates a bouncing ball away
from the table or propels a fired bullet down the barrel of the gun.


If I reconstruct Ron's concern properly, he sees people saying that
Newton's third law explains bullets and rockets and bouncing balls.
But he doesn't see anything in Newton's third law to explain these
"excess" forces.

Of course he's perfectly correct in this.

The "excess" force on the bouncing ball is due to an an elastic deformation
of rubber molecules -- the ball is stressed and deformed out of its
normal spherical shape. Newton's third law is silent on the nature of
the deformation and stresses in the rubber ball. Hooke's law may
approximately apply.

The "excess" force on the bullet is due to high temperature, high
pressure gas in the firing chamber. Newton's third law is silent
on the nature of the temperature, pressure and resulting forces.
Boyle's law may approximately apply.
.