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Re: [escepticos] Inercia

----- Original Message -----
From: "Eloy Anguiano" <eloyang en teleline.es>
To: <escepticos en ccdis.dis.ulpgc.es>
Sent: Friday, December 14, 2001 6:35 PM
Subject: Re: [escepticos] Inercia

> Javier Susaeta wrote:
> > ++++++++++++
> > Creo que sí tiene. Si no, el experimento -mental o material- del 'cubo
> > giratorio de Newton' no tiene otro recurso que el 'espacio absoluto'.
> > +++++++++++
> ¿A qué experimento te refieres?


A este que mando como 'attachment', que lo he sacado vía 'Google', buscando
por 'absolute space'. El que yo recordaba era el de un cubo lleno de agua,
girando sobre su eje, colgado de una cuerda. La superficie tomaba la forma
de un paraboloide, lo que medía la velocidad angular, ¿respecto a qué?

Aquí son dos bolas unidas por una cuerda, pero la pregunta es la misma.
¿Respecto a qué?



Newton clearly understood that it was impossible in any absolute way to 
determine motion at constant velocity. Motion at constant velocity is meaningful 
only as relative motion since there is no experiment you can do to determine if 
you are moving at constant velocity or ``at rest.'' But Newton believed in 
absolute space relative to which all motion occurred. Newton knew that one 
couldn't determine motion at constant velocity relative to absolute space. On 
the other hand, he said that it is possible to determine accelerated motion 
relative to absolute space. In other words, accelerated motion is absolute. One 
example Newton gives to illustrate this point involves an experiment in which 
two globes are connected by a string and set in circular motion about the center 
of the string. Imagine you are in a totally white room so you have no reference 
to see whether or not the globes are rotating. Newton gives a simple 
prescription for telling if the spheres are rotating about one another. Cut the 
string! If the balls fly apart, they were rotating; if they just sit there, they 
were not rotating. The tension in the string is a measure of rotation relative 
to absolute space. 
Newton's notion of absolute space was questioned by Mach around 1900. Mach gives 
a somewhat different scenario. Mach postulates that only relative motion has any 
significance. How then does he explain Newton's experiment. Mach states that the 
spheres are rotating relative to the fixed stars, which contain most of the mass 
in the Universe. If we were able to leave the balls fixed and rotate the stars 
in the opposite direction, the relative motion would be the same. According to 
Mach, since the relative motion is the same for the two cases and since there 
was a tension in the first case (when the spheres were rotating and the stars 
fixed), there must be a tension in the second case as well. Of course, according 
to Newton, there would be no tension in the second case since the spheres are 
not rotating relative to absolute space. 
Who's right? Newton or Mach? Einstein was influenced by Mach and believed that 
Mach's Principle would be incorporated in his General Theory of Relativity. It 
turns out that Mach's Principle is not incorporated into the General Theory, 
although there is a small residual effect. This is an experimental question and 
the final verdict is still not in, but it appears that Newton's view is closer 
to reality than Mach's. 
A technical point. Often when I ask students to give an example to show whether 
Newton or Mach is right, some students say ``Take two balls on a string and 
rotate them in a room - there is tension in the string. Then leave the balls 
stationary and rotate the room in the opposite direction. If there is tension in 
the string Mach is right.'' This reasoning is wrong, however. The entire 
relative motion of the spheres to the Universe must be the same. Since most of 
the mass of the Universe is in the fixed stars, it is the motion relative to the 
fixed stars that is relevant.