Re: Barn-Pole Nuts & Bolts contradiction

On Tue, 28 Aug 2007 06:13:42 -0700, "N:dlzc D:aol T:com \(dlzc\)"
<dlzc1@xxxxxxx> wrote:

Dear David:

"David" <dseppala@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.

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.

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.

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. If the nut has 100 threads that means
as measured in the nut's frame 100 peaks and 100 valleys at each
instant of time are aligned on a line parallel to the longitudinal
length of the nut on the nut's interior surface. If the threaded rod
also has 100 threads that means that as measured in the nut's frame
the threaded rod also has 100 peaks and 100 valleys aligned along the
longitudinal length of the rod at any point in time. For the threaded
rod and screw to fit perfectly, each peak must be aligned with a
valley. But SR says that the 100 peaks and 100 valleys of the
threaded rod span a longitudinal distance greater than the 100 peaks
and 100 valleys of the threaded nut. This is contradictory - its a
simple counting exercise.
David

David A. Smith

.

Relevant Pages

  • Re: Barn-Pole Nuts & Bolts contradiction
    ... by a long threaded nut and the pole replaced by a long threaded rod. ... length L as measured in a rest frame. ... apply to a nut that looks additionally twisted due to motion: ...
    (sci.physics.relativity)
  • Re: Barn-Pole Nuts & Bolts contradiction
    ... by a long threaded nut and the pole replaced by a long threaded rod. ... length L as measured in a rest frame. ... OK, so they are equally contracted as measured in this rest frame, so yes ...
    (sci.physics.relativity)
  • Re: Drill press depth stop
    ... Is the nut in 2 pieces that jam together to stop ... The only ones I've used with the threaded rod had NO form of quick- ... that would make things a little better with one drill press, ... It never loosens up in the middle of ...
    (rec.crafts.metalworking)
  • Re: Drill press depth stop
    ... Is the nut in 2 pieces that jam together to stop ... The only ones I've used with the threaded rod had NO form of quick- ... It never loosens up in the middle of ... release the button and then turn the nut for fine adjustment. ...
    (rec.crafts.metalworking)