On 26 Aug 2005 07:33:58 -0700, "Spoonfed"
wrote:


wrote:
On 23 Aug 2005 11:07:01 -0700, "Spoonfed"
wrote:


wrote:

We recognize that sight and sound can be missleading and that no wrapping
actually occurs.

Wrapping actually does occur. In the rest frame of the rod and of the
rotating cylinder, we are attaching one end of the rod to the cylinder
a second before we attach the other end. The cylinder rotates 10
times before the other end is attached. Since the speeds are everyday
values can actually do this experiment over a much shorter length.
The short rod will attach to the short segment of the rotating disk,
and if we attach one end at a different time then we attach the other
end (the stationary frame view), the attached rod will spiral about
the cylinder. In the problem I posted because of the lengths involved
the spiral and wrapping occur over large distances, but it still
occurs.
David

Right. If either of the two observers can be said to be measuring an
illusion, it is the non-local observer, who measures the rod to be
straight on a moving, length contracted, rotating cylinder.

The geometry and forces which are actually holding the rod to the
cylinder are local effects, and the rigidity of the coiled rod in the
stationary frame.

Aside: The nonlocal observer will only "see" the rod on the cylinder
as straight if and when the rod passes directly in front of him, on a
path perpendicular to his line of sight. Otherwise, differences in
light propagation times between the front and back of the rotating
cylinder will result in an image that does not perfectly represent the
"present" in his frame of reference.

When removing the rod/coil from the cylinder, you want to have a clamp
that comes down on all portions of the rod at once in the nonlocal
frame, and pulls it away from the cylinder with an instantaneous YOINK!

What the clamp will look like from the rest frame is more like a
zipper. In the stationary frame, the two edges of the clamp will not
come down toward each other parallel, but at an angle, meeting at a
point on the rod, gripping it, pulling it loose. The point would roll
up the clamp edges as it passed by, pulling the rod loose as it goes.

Since we have stiffened the rod, it will take considerably more force
to remove the coiled rod than it took to put it on there. Quite
possibly the clamp would simply fail to remove the the rod from the
rotating cylinder, slipping off the rod instead of grabbing hold of it.
I think your conundrum is that the rod appears to be straight in the
nonlocal frame, and it has not been glued to the cylinder; it has not
been increased in mass--it has only been stiffened. Why would
stiffening a straight rod cause it to be harder to remove from a
cylinder?

Yes, so what is the explanation for observers in the moving frame,
when all the contact points holding the rod to the cylinder are
removed simultaneously? They do not measure any force on the rod, and
therefore it should be a straight rod simply traveling off in the
direction of the velocity it had when the contact points were
eliminated. But that's not what happens in the stationary frame of
the rotating cylinder and rod.
David

You say the moving observers do NOT measure any force on the rod, but,
I am saying there WILL be a measurable force holding the straight rod
against the cylinder in the moving frame.

After thinking some more, do you agree with me, now?
I agree there is a measurable force holding the straight rod against
the cylinder in the moving frame, but when each point of contact is
eliminated at time t1 as measured in the moving frame, there is no
longer any force acting on the rod. Each point of the rod continues
in the direction it was traveling at the instance the point of contact
was eliminated. The rod should remain a straight line as measured in
the moving frame after each point of contact is eliminated.
David

My bad copy/paste work, and bad joke may have caused you to miss my
last two paragraphs above:

[Jesting deleted]
I think the answer lies in the nature of the
collisions--where the rubber meets the road, so to speak. If you look
closely at the interactions and forces between the particles in the
rod/coil, and in the clamp/zipper (in either frame), you'll find that
they are more tightly bound to the cylinder after the rod has been
stiffened.

The analysis of exactly how the apparently straight rope attaches
itself in the moving frame is still probably a fairly intricate
problem.