Re: Weirdo's square of negative mass.

In article <m62qd5t23bm14snj9ra3gucsckem40e4ds@xxxxxxx>,
Bob Casanova <nospam@xxxxxxxx> wrote:

On Mon, 19 Oct 2009 23:31:09 +0200, the following appeared
in talk.origins, posted by nospam@xxxxxxxxxxxxxxxx (J. J.
Lodder):

Prof Weird <poland@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Oct 19, 12:13 pm, Paul J Gans <g...@xxxxxxxxx> wrote:
Prof Weird <pol...@xxxxxxxxxxxxxxxxxxxxx> wrote:
On Oct 19, 7:58 am, spintronic <spintro...@xxxxxxxxxxx> wrote:
Professor weirdo & william have come out with an ingenious way to
create negative mass.

They claim a photon imparts momentum twice to a body it is deflected
from.
That only applies where the mass reflected from is much heavier than
the photons coming in. It is a slight overestimate, since there must
be less momentum reflecting than continuing on.
The mirror would have momentum pi+ pr, while the photons that are
reflected back have momentum -pr.
Thus the ACTUAL ratio is (pi + pr)/pi, which APPROACHES 2.0 as the
difference in mass between the mirror and the photons increases.

It ain't just photons. A ball hitting an immobile wall also
imparts twice its momentum to the wall, assuming that the ball
is perfectly elastic, of course.

There's nothing magic about photons per se.

--
--- Paul J. Gans

True - its an effect of conservation of momentum.

If ball/photon has momentum p going right, then hits wall, the wall
gains momentum p.
When the ball/photon reflects, it is now going LEFT (has -p
(reflection), and wall gains p(reflection).

Since momentum has been transferred, p(reflection) must be less than p
to conserve energy.

So ends up :

Before : ball/photon has momentum p, wall has p2 = 0, for a total
momentum of p.

After : ball/photon has momentum -pr, wall has p2 = p+ pr. System has
a total momentum of p, and the wall has gained momentum p + pr.

If the wall is much more massive than its impactor, it looks like it
gets 2p momentum (actually gets something like (2 - 2*m1/mT)p, where
m1 = mass of impactor and mT = mass of impactor + wall) for
sufficiently small m1/mT.

If the wall is 1,000,000 x heavier than the ball, wall gets about 2-
(2/1,000,001)p, or 1.999998 p or so.

If the wall is immobile (ie, close to infinite weight), then it gets
about 2-(2/infinity)p, or as close to 2p as you can get.

But finding truly immobile walls and perfectly elastic balls it rather
difficult .. ;D

As are frictionless pulleys, massless ropes, etc.

It's called idealization,

And good teaching aids, similar to zero-resistance wires and
lossless inductor cores. And one doesn't (or shouldn't)
start teaching orbital mechanics using relativity.

We have to break it to the students gently so they can suspend
disbelief. I remember the incredulity some of my fellow students
expressed over the introduction of Newtonian Physics in High School.

--
A computer without Microsoft is like a chocolate cake without mustard.

.

Relevant Pages

  • Re: Weirdos square of negative mass.
    ... be less momentum reflecting than continuing on. ... The mirror would have momentum pi+ pr, while the photons that are ... A ball hitting an immobile wall also ...
    (talk.origins)
  • Re: Weirdos square of negative mass.
    ... be less momentum reflecting than continuing on. ... The mirror would have momentum pi+ pr, while the photons that are ... A ball hitting an immobile wall also ...
    (talk.origins)
  • Re: Weirdos square of negative mass.
    ... be less momentum reflecting than continuing on. ... The mirror would have momentum pi+ pr, while the photons that are ... A ball hitting an immobile wall also ...
    (talk.origins)
  • Re: Weirdos square of negative mass.
    ... be less momentum reflecting than continuing on. ... The mirror would have momentum pi+ pr, while the photons that are ... A ball hitting an immobile wall also ...
    (talk.origins)
  • Re: Weirdos square of negative mass.
    ... be less momentum reflecting than continuing on. ... The mirror would have momentum pi+ pr, while the photons that are ... A ball hitting an immobile wall also ...
    (talk.origins)