Re: [Phys-L] relativistic acceleration of an extended object

[John Denker <jsd@av8n.com>]

On 11/01/2014 09:51 AM, Moses Fayngold wrote:
> But I have some comment on the basic statement in the text itself:
> "...in order for the object to maintain its shape, different parts
> will need to accelerate at different rates".
>
> This statement is ambiguous. Its truth value depends on the chosen
> reference frame (RF) and on definition of "shape".

If you would read the article, you would find the answers
to those questions.  Acceleration means proper acceleration
at each point.  Shape is determined by proper length, measured 
along a contour of constant time.  All observers agree that 
the contour in question *is* a contour of constant time, so
there is no ambiguity whatsoever.

>  Let us define the
> shape as an instantaneous configuration of the object in a given RF.
> Then it is easy to see that in the initial rest frame of the object
> (frame A), the statement is wrong. 

It's not wrong. 

> Different acceleration rates for
> different parts will surely distort the shape.

False.  Read the article already.

>  In order to conserve
> it in A, we need the same acceleration a for all parts.

False    (except perhaps in the non-relativistic limit, which
is not what we are talking about here).

Would it kill ya to read the article?