Re: basic mechanics question


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?

>
> > 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?

>
> > 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.

>
> Jim G
> c'=c+v
> (along with clocks atop axles which don't keep the same time as those
> at ground, yet do not twist the axle)

.

Relevant Pages

  • THE COMPOSITION OF VELOCITIES
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  • addition of velocities
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    (sci.physics.relativity)
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    (sci.physics.relativity)
  • Re: The Addition Operator Absent in Relatvity !!!
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    (sci.physics.relativity)