Re: Summarry of the momentum wars and idea.

[Steve VanDevender <stevev@efn.org>]

KellySt@aol.com writes:
 > >> Besides not treating momentum as a vector quantity, people are
 > >> making the mistake of thinking that lateral loading of the sail
 > >> assembly is a magical sink for momentum or energy.  The error is
 > >> in thinking that stress on a static structure absorbs energy or
 > >> momentum continuously over time.  If the sail does not move
 > >> relative to the ship, then it cannot absorb or dissipate momentum
 > >> separately from the ship.  It cannot absorb momentum if it does
 > >> not move, because momentum means motion.  ----
 > 
 > (??!)  <sigh>
 > Thats like the old argument that if a tractor is pushing against a wall its
 > doing no work, since the wall isn't accelerated.  The sail is getting a
 > thrust that is perpendicular to the surface of reflection.  If you want to
 > describe the portion of the thrust that isn't accelerating the ship as
 > invalid, enjoy.

The tractor is dissipating energy because it contains moving
parts in its engine and drive train that keep moving even though
the tractor chassis and the wall are not.  And it did do some
amount of work compressing itself against the wall; when the
engine is turned off, the tractor will roll away from the wall.

Lean a board against the wall.  Does it dissipate energy because
it can't move the wall?  Lean a heavy iron bar against the wall.
Does it dissipate more energy than the board?  Are the bricks at
the bottom of the wall permanently warmer than the bricks at the
top because they are under compression?

These are all what I mean by static stress.  Static stress does
not dissipate energy.  Varying stress dissipates energy; the
tractor engine powered by intermittent gasoline explosions
produces vibration and heat as it pushes the tractor against the
wall; the tractor vibrating against the wall even heats the wall.

On the other hand, the board or the iron bar, or the bricks at
the bottom of the wall, do not dissipate energy.  If you think
they do, tell me where it comes from.  You can't claim that
gravity is continuously pumping energy into the objects; you
can't gain or lose energy if you don't move up or down in a
gravity field.

 > But when you start to mutter things like:
 > >> ---as long as the sail does not fall apart or the support 
 > >> members do not break, no more energy is dissipated into 
 > >> loading of the sail structure.
 > 
 > We have a problem.  
 >
 > A considerable amount of energy will be continuously loaded and (hopefully)
 > disapated by the sail cross webbing.  If we don't consider it, and make sure
 > the structure can disapate it, the cross cables will melt under the energy
 > they have to disapate under this lateral thrust load.

The structure will dissipate energy if the photon beam varies
periodically with time, as the up-and-down variations will induce
vibration in the structure.  This is certainly a real engineering
problem that would have to be considered in a real ship.

My intention was to prove that Kevin's parasail design couldn't
absorb photons without absorbing their momentum.  This principle
is still true even if the parasail isn't exposed to a completely
steady, unvarying photon flux.  The members of the support
structure don't "dissipate" or "absorb" forward momentum because
they are under sideways load.

You seem to have a real misunderstanding of the difference
between work and potential.  When you put a structure under
tension or compression, you do change its energy, ONCE, when you
slightly pull apart or scrunch together all of the atomic bonds
in the object; you have put potential energy into the object.  As
long as the forces on the structure do not change, no further
energy flows in or out of it; it does not continue to dissipate
energy with time.  Your house does not radiate heat because it is
in a gravity field.  The cornerstone of the Empire State Building
is not hot because it is holding up the weight of the building.
The Earth is not losing energy to everything that its gravity
holds to the ground.

Just as I could build a reactionless drive from Kevin's parasail,
if I play by your rules I can build a perpetual motion machine
from a piece of metal in a vise; all I have to do is extract the
energy that you claim it will continually radiate for as long as
it is in the vise.

The reason that real engineering structures dissipate energy is
that there is rarely such a thing as pure static stress.
Structures endure varying loads -- cars drive over bridges, the
Empire State Building sways in the wind and absorbs millions of
footsteps.  The cornerstone of the Empire State Building isn't
warmer because it's under compression; it's warmer because it
dissipates energy from swaying and vibration.

 > One very consistent problem in LIT over the last year has been
 > a very limited interest in the engineering realities of a
 > situation, and to much fondness for endless equation wars.

These are not equation wars; as this message shows, you don't
always have to use math to talk about physics.  We are talking
about very real physics concepts, and I'm afraid you are the one
who has a few important ones wrong.

Before we can do the detailed engineering for a starship, we need
to understand what physical constraints it will be under.  Even
the best engineering cannot violate the laws of physics.  If we
are engaged in "equation wars", it is because we are trying to
figure out the limits of what is possible before we do detailed
design work based on faulty assumptions.  This is the most
fundamental interest in engineering realities you could want.