Re: Barn-Pole Nuts & Bolts contradiction

On Aug 28, 7:52 am, David <dsepp...@xxxxxxxxxxxxx> wrote:
On Tue, 28 Aug 2007 06:13:42 -0700, "N:dlzcD:aol T:com \(dlzc\)"

<dl...@xxxxxxx> wrote:
Dear David:

"David" <dsepp...@xxxxxxxxxxxxx> wrote in message
news:5p58d3d6aiubpjio3g828vkldtfp3j6ej7@xxxxxxxxxx
This problem is the classic Barn Pole paradox with
the barn replaced by a long threaded nut and the
pole replaced by a long threaded rod.

Let there be a threaded nut of length L and a
threaded rod also of length L as measured in a
rest frame. Let each of these have N threads.
Now let the threaded rod and threaded nut move
toward each other with equal and opposite
velocities as measured in this rest frame. And
let the threaded nut and the threaded rod each
rotate such that when they meet the rotation
rate is such that threaded nut and threaded rod
fit perfectly and neither the rod nor the nut
undergo any acceleration.

How do you have rotation without acceleration,
David?

From the context, I thought everyone would have
known I meant linear acceleration. I should
have been more precise in the phrasing so as
not to confuse others.

Acceleration is acceleration, David. Linear or not. You are already
in a realm where SR does explicitly not apply. It may be reasonably
*extended* to this problem... but if you are speaking to posterity,
you *are* confusing the issue.

How is this viewed using SR in the rest
frame of the nut?

Note that for any appreciable axial velocity,
the tangential velocity will be many times
greater... depending on the pitch.

Ignored.

In this frame, per SR, the threaded
rod spans a shorter distance than the
threaded nut.

No, because the surface of the threaded
rod is moving at an angle to the nut's
central axis. You are ignoring the meshing
surface, while concentrating on the "center
of mass".

Here, again, I mean the longitudinal length
of the rod versus the longitudinal length of
the nut.

Here again, you ignore a major component of velocity, which is where
length contraction occurs. It occurs along this axis of motion, which
is NOT parallel to the motion of the center of mass.

So that when the rod is completely
encased by the nut,as viewed in the
inertial frame of the nut, the N threads
of the rod span less than the N threads
of the nut. I don't see how this is
physically possible. Can someone
explain this using SR concepts.

SR doesn't do acceleration simply enough
that you can understand, David. It seems
to keep leaking out of your head.

Perhaps not, but I can count.

You ignore the *total* velocity of the very surface you want to
"observe" length contraction in.

<rest snipped because you sail off on a tangent again)

The length contraction of the rod, in the rest frame of the nut,
occurs parallel with the "axis" of a single thread... for each
thread. No length contraction is seen to occur between pitches,
because the total motion *of the mating surface* is perpendicular to
the pitch "normal".

<READ THIS DAVID, if you read nothing else>
Look at the meshing of two adjacent threads as they enter the nut.
How does the meshing surface of the rod appear to move here? It moves
parallel to the threads, and you know there is *no* length contraction
perpendicular to the axis of motion.
<END OF READ THIS DAVID>

David A. Smith

.

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