Re: basic mechanics question


jgreenfield@xxxxxxxxxxx wrote:
> PD wrote:
> > jgreenfield@xxxxxxxxxxx wrote:
> > > PD wrote:
> > > > jgreenfield@xxxxxxxxxxx wrote:
> > > > > PD wrote:
> > > > > > jgreenfield@xxxxxxxxxxx wrote:
> > > > > > > vsgdp wrote:
> > > > > > > > Hi,
> > > > > > > >
> > > > > > > > Suppose gravity does not exist.
> > > > > > > >
> > > > > > > > A car is traving on a frictionless horizontal plane to the right at some
> > > > > > > > velocity v.
> > > > > > > >
> > > > > > > > Up ahead there is a ramp (no gravity).
> > > > > > > >
> > > > > > > > When the car hits the ramp, is it correct to say that some of the velocity
> > > > > > > > is lost due to the impulse. And the remaining velocity is the projection of
> > > > > > > > v onto the tangent vector of the ramp?
> > > > > > >
> > > > > > >
> > > > > > > Newton: "Energy is neither created nor destroyed, but may change from
> > > > > > > one form to another". So the car alters its direction (and so does the
> > > > > > > ramp minisculely), loses linear speed (gives up kinetic energy) and
> > > > > > > heat is "generated"/shared between the ramp and car from the "lost" KE.
> > > > > > >
> > > > > > > Now think on to the "adding velocities under SR rules".
> > > > > > > Using the adopted method of SR, the CHANGES in DIRECTION of the objects
> > > > > > > whose velocities are to be added, and the inherent transfers of energy
> > > > > > > therein as visualised in this thought experiment, are IGNORED.
> > > > > > > Such "ignorance" falsifies the theory, as the method of addition which
> > > > > > > underlies it is bogus.
> > > > > > >
> > > > > >
> > > > > > And this is crap.
> > > > > > 1. Relativistic addition of velocity in more than 1 dimension is
> > > > > > perfectly well-defined, though you may not have seen that.
> > > > >
> > > > > Correct! All I see is the King's hairy arse; no magic cloak.
> > > > > I am of course referring to additions of velocities for objects on the
> > > > > x axis
> > > > > (a car firing a bullet ahead). SR addition blythely introduces a
> > > > > "slope" for the path of the bullet, totally ignoring the energy needed
> > > > > to produce the change in direction.
> > > >
> > > > What slope would that be? Citation please. I believe you are
> > > > misunderstanding whatever you are reading. Ah, wait, let me guess.
> > > > You're looking at the slope of a worldline on an x vs t graph? And you
> > > > can't distinguish the physical meaning of that from an x vs y graph?
> > >
> > > Yes. I am looking at a slope which has another triangle placed upon it,
> > > which viewed in its own frame, would still have its base on the x axis,
> > > NOT transposed to (rotated) lie on the "slope".
> >
> > Sorry, this didn't help. Unintelligible.
> >
> > > To plot velocity, which
> > > is ALREADY a function of time, against another "axis" of time,
> >
> > *Another* axis? Not sure what you mean. Even classically, I can plot
> > velocity vs. time. This is how I tell whether an object is
> > accelerating. What's wrong with that?
>
> Nary a mention of acceleration in this scenario; just plain addition.

Still unintelligible. I don't know what you're looking at. Reference
please.

> You may plot SPEED against time, but NOT velocity, because how is the
> direction changing?

Er, the direction of a velocity is determined by its components. So
three plots of velocity components versus time will certainly tell you
how the direction is changing and therefore what the acceleration is.
Whatever are you going on about?

> Hint: It isn't, and the "slopes" used in Relativistic addition of
> velocities are bogus.

I don't know what "slopes" you are babbling about. Reference please.

> They represent an unfunded (by energy) change to direction---end of
> story.

I *believe* you don't know what you're looking at, but I'll take a wild
guess.
Both SR and Galilean relativity allow that an object's velocity --
including its direction -- will change if a different reference frame
is chosen. This in *no way* implies non-conservation of energy or
momentum. I'll give you a simple example from conventional physics so
that you can see what I'm talking about.

Consider an airplane, American Airlines flight 732, flying at 400 mph
to the northeast, that velocity measured with respect to the ground.
Consider another airplane, Continental 1721, flying due north at 565
mph at higher altitude, that velocity also measured with respect to the
ground. A passenger on Continental 1721 looks out the window and sees
American 732 pass under their plane. This passenger will see American
flight 732 flying at 400 mph but to the *southeast* with respect to
themselves. A little straightforward vector math will bear this answer
out. This change in reference frame in NO WAY implies that something
happened to the American plane to change it from going northeast to
southeast.

> >
> > > leaves
> > > situation of plotting time against itself, which may be ok for the
> > > gullible. Of course, Srists think it ok, as they apriori equip
> > > different objects with clocks which tick at a rate which protects "The
> > > Theory", and the two "times" being different, accept the graph as seen.
> >
> > Still unintelligible. Do you have a reference?
>
> a few years being ridiculed for NOT accepting that clocks "are what
> measure time, and tick according to their velocity"

Is that your reference, or are you looking at something in a book?

> >
> > > >
> > > > >
> > > > > > 2. Conservation of momentum is not tantamount to a velocity addition.
> > > > >
> > > > > A convenient cop out (re-definition) to protect SR
> > > >
> > > > Bull. A baseball is pitched toward homeplate at 90 mph. The batter
> > > > swings forward, the end of the bat moving at 60 mph just before
> > > > contact. After contact, the bat continues forward at 59 mph, and the
> > > > baseball heads toward the outfield at 209 mph. All this is completely
> > > > consistent with conservation of momentum. Where is the velocity
> > > > addition here?
> > >
> > > This may be fine for low speed, but SR says if the bat is travelling at
> > > relatavistic speed, and the ball also, then the combined (added) speed
> > > is less (.5c +.5c = <c)
> > > Given perfect elasticity (no heat exchanges), whence the lost
> > > momentum???????
> >
> > You missed the point. SR does not *add anything more* to momentum
> > conservation. The rules that apply to the baseball above are
> > *identical* to the rules that would apply in a relativistic treatment.
> > Velocity addition has *nothing* to do with the change in velocity that
> > is associated with an interaction. Velocity addition has to do with the
> > change in velocity associated with a change in reference frame
> > *without* consideration of an interaction. If you are confused on this
> > point, then you do not understand this part of SR at all.
>
> Does or does NOT .5v + .5v = <1v when v = c ??????????????

Poorly posed question. To combine velocities, you do not use a +.
This is as poorly posed as asking whether 1 cup of water + 1 cup of
flour = 2 cups of paste. The answer is no, because the volumes of water
and flour do not add that way, whether you think it should or not. The
same is true for velocities. To combine velocities, you may NOT use a +
sign. You must instead use (v1 + v2)/sqrt(1 + v1*v2/c^2).

Well, I take that back. You *can* use v1 + v2, and in some cases it
will be a good approximation, just like you *can* use 1 cup + 1 cup and
to some approximation it will be ok.

> >
> > > >
> > > > >
> > > > > > 3. Conservation of momentum (including collisions in more than one
> > > > > > dimension) is strictly enforced in special relativity.
> > > > >
> > > > > With distances per time that alter magically depending on the speed of
> > > > > the object, how could it fail?
> > > >
> > > > Failure to understand SR and conservation of momentum, and subsequent
> > > > of same as "magic", is not a valid critique.
> > >
> > > c = fu
> > > when f changes, or u changes, WHY SHOULD the other alter to maintain c
> > > the same??????????
> > > THAT is MAGIC
> >
> > Um, the same reason that when you sing A instead of C (thereby changing
> > the frequency), the wavelength also changes. Is that magic?
>
> It is magic for you to tell how fast I am moving ref you. How do you
> know WHICH note I am singing????????

Ah, because you have declared you're singing Amazing Grace in D, which
is a tune I recognize. And when I look at a star, I recognize the tune
"calcium" and so I *know* what note the star is singing, because all
stars sing "calcium" the same way.

> Besides which, if the comparison is to be valid for a shockwave through
> air (sound) to be equivalent

But they're *not* equivalent. That's why the expressions for the amount
of Doppler shift is different in the two cases, even though the
*principle* is similar.

> to light through space, then the statement
> becomes unequivocal that SR depends on a medium, and ergo is an AETHER
> (jello) theory.
> It is "time" that you grasped that doppler is an effect,

Yes, but it is not a *single* effect that happens the same way in all
cases. It is a *family* of related effects. It is "time" you grasped
that, perhaps by looking at a book.

> NOT a cause,
> and think on the implications.
>
> Jim G
> c'=c+v

.

Relevant Pages

  • Re: Experimental Detection of Universal Reference Frame
    ... >> The Principle of Relativity is invalid precisely on the ground that it ... >> declares all inertial reference frames in relative uniform motion to be ... >> physical effects without reference to any preferred reference frame. ... >> uniform velocity with respect to BCRF. ...
    (sci.physics.relativity)
  • Re: AdjOPS versus W/G scatter plot for 2009
    ... At the velocity of light, ... of relative motions and one's own inertial frame of reference (i.e. ... Kepler's Second Law is that the radius from the focus to a planet ...
    (alt.sports.baseball.bos-redsox)
  • Re: Galilean transformation explanation of MMX
    ... as A and B. A light signal is emitted from A, travels to B, is reflected ... frame of reference relative to which the rod moves, ... the velocity of the light is w=c. ...
    (sci.physics.relativity)
  • Re: Query about simultaneous events..
    ... >> frame of reference of N', the light is emitted at A' and B', not at A ... >> velocity of light, not speed. ... Why would it be different for a photon? ...
    (sci.physics.relativity)
  • Re: Reviewing Statements in Einsteins 1905 paper.
    ... |> they have a relative velocity with. ... |> your reference but you have to tell me what it is so that I know ... By casting the shadow of an ellipse onto a plane surface ... Tell the Pope his god nailed to a tree was not born of a virgin. ...
    (sci.physics.relativity)