Re: About absolute reference frame......


Mike wrote:
PD wrote:
Mike wrote:
PD wrote:
Mike wrote:
Tom Roberts wrote:
Mike wrote:
Tom Roberts wrote:
Newton's second law is F=ma, and in a rotating frame that
simply does not hold -- one must _invent_ fictitious "forces" (called
"centrifugal force" and "Coriolis force") in order to "save" the law.

These forces are not ficticious at all. This name is a historical
misnomer.

Applying the word "force" to them is the misnomer, not the word
"fictitious".


Are you trying to deny the real existence of centrifugla
forces that make centrifuges work and coriolis forces who are
responsible for the rotation of the water when it goes down the drain?

But they don't. <shrug>

Centrifuges work because the material in them tries to move in a
straight line (relative to the inertial frame of the centrifuge's
center), but the centripetal force of the centrifuge outer wall compels
the material to deviate from that straight line and move in a circle.
The centripetal force ("center seeking") is real, and is what causes the
material to move in a circle.


Just because something is coordinate dependent it
does not mean it is not "real".

Sure it does! Arbitrary human choices of how to describe a real
phenomenon cannot possibly affect the phenomenon itself.


Calling the last two terms in my equation above "forces" does not make
them so. <shrug>

It seems that in order for you to support you false argument you will
go out of your way calling forces that everyone feels every day in a
car, in an amusement park, riding a bicycle, unreal.

You have _NEVER_ felt "centrifugal force". What you have felt is the
door of a car pushing sideways on you as the car turns a corner. Etc.
All of these "fictitious forces" have that character -- nobody _ever_
feels them (because your nerves sense differences, and these "forces"
are all proportional to mass and thus there is no difference for your
nerves to feel; what you feel is other forces which are _always_ in the
opposite direction: your chair pushes up, the car door pushes toward the
_inside_ of the curve, etc.).


Besides containing several innacuracies and basic physics errors, your
neurophysiologicallly based assertions to defend an absurd postion show
desperation.

I suggest to you getting a ride in one of those holocoasters. Sure, you
will feel getting pushed against the side of your cart. But when you
feel your lips and cheeks getting tweested you will understand the
power of the centrifugal force.

When you accelerate forward in your car, the little voodoo head hanging
from your rear-view mirror by a string swings backwards. Why does it do
that?

I'm glad you've notices that.




One person might say there is a force backwards on the voodoo head. Of
course, one is hard pressed to find the agent of such a backwards
force. What would be pushing backwards?

Agent 007


Another person might say that in order for the voodoo head to keep up
with the car, a force has to be applied to it to make it accelerate
forward. The only agent available for such a force is the string.
Unfortunately, a string can only pull along its length and so a
vertical string cannot possibly apply a horizontal force required for
horizontal acceleration. Now, if the string were to be inclined, so
that its length were partially vertical and partially horizontal...

Someone is "pulling the strings then". Some "agent"...

Which explanation makes more sense?

The explanation I gave before in the case of the marry-go-around
experiment which I suggest you read it and understand it.

If you and Roberts have any doubts theat a coriolis force for example
is a real force all you have to do is watch the water go down the sink
drain. Actually its rotation direction is different in the north and
south hemisphere, this shows that water molecules are clearly affected
by this real force which Einstein misunderstood and ended up
transforming away.

Actually, Einstein did no such thing. He didn't have a lot of comment
about the Coriolis force.

As for whether the force is real or not, I suggest you take a friend to
the playground for an experiment. Sit on a merry-go-round and toss a
playground ball across the rotating platform while you stand on it.
You'll need a third person standing on the ground to observe the motion
of the ball through the air and ask whether this person sees any force
acting on the ball.

if you want to discuss physics seriously you must first understand that
forces are measured, not 'seen". Then:

I disagree that they are measured. Forces are *deduced* by a
combination of
- measured effect, usually by measuring acceleration (or lack of it)
- being able to attribute an agent of that force, a partner in the
interaction.
Having one without the other is not sufficient.



Attach a spring to the center of a rotating top and a steel ball at the
end of the spring. set the top in constant rotation. You will now
measure a force on the ball although it is not moving. For an observer
in an in ertial reference frame (if such thing really exists and that
is a small part of the confusion) everything is explained by Newton's
2nd law. The force on the ball is the centripetal force.

Now, the confusion you and Roberts has and Einstein did of course is
what happens in the case of an observer moving with the ball. the ball
appears stationary but there is a force on it as indicated by the
spring. is this a violation of newton's law? Do we need ficticious
forces to correct for such observation as many naive thinkers believe?

Both answers are negative. Now, we have a different system all together
to consider and the mistake Einstein did and you and Roberts do along
with many many others is that the same analysis does not hold. The
centrifugal fprce which is real and is impressed on the ball did not
need to be considered in the case of the inertial FoR because it is an
internal force acting on the pin that holds the spring attached to the
rotating top. But when you consider the frame moving with the top, this
force must be translated and added to the forces acting on the ball
because the pin is part of the inertial frame that holds the top
rotating. Thus, newton's law holds again and the sum of the forces on
the ball is zero and you get no change in motion, as expected.

An inertial frame is *defined* as one where the first law holds --
where a net force is absent, there is no acceleration. This is how one
can tell if the the frame of reference is not an inertial frame: that
there is acceleration where there is no net force, or where there is no
acceleration where there is a net force. Keep in mind that a the
deduction of a force stems from both qualities above, and so an agent
or a partner in the interaction must be identifiable.


The dispute is whether centrigugal forces are real or ficticious. I
have shown to you that they are real forces, and produce real change in
motion. Now, transforming away real forces because one thinks they are
ficticious is not a good idea and leads to absusrd physical models.
That is the issue. So basically, the laws of mechanics are the same in
both frames and all forces are real. But the two frames are different
by definition. making them the same as Einstein attempted with the
strong version of the POR is a twisted view at reality.

Now, what remains is to ask whether the difference in the two analyses
constituttes a violation of the epistemological POR. It does not, since
the two frames are not equivalent.

All the merry-go-around and cameras in the world will not teach physics
to anybody who cannot formulate and solve a simple problem according to
thr rules of mechanics, which involves a simultaneous understanding of
statics, kinematics and dynamics.

And someone who does not understand what is going on with the video of
the merry-go-round is unlikely to be able to formulate or solve a
simple problem according to the rules of mechanics.


Mike





http://archive.ncsa.uiuc.edu/Cyberia/DVE/FusionDVE/html/coriolis_force_lesson_plan.html







PD

.

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