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Re: [Phys-L] first/second/third law



On 06/25/2013 12:30 PM, Bruce Sherwood wrote:
... I've sometimes
heard it claimed that the first law is to be taken as "there exist
reference frames in which the first law holds" or some such.

That is a very deep question. There was a time, from the 17th
century through the 19th century inclusive, when the first law
did stand on its own, as a prerequisite for the second law.

However, the modern (post 1915) approach says we can use *geometry*
to define what we mean by a straight line through spacetime. This
is waaaay better than using the first law for this purpose.

It takes some people hours or even days to come to grips with
this, but it's worth the trouble. Perhaps the best place to
start is with a hands-on procedure for constructing geodesics
on a tabletop:
http://www.av8n.com/physics/geodesics.htm
This is easy and fun. It's something that I can explain to
9th-grade students. (Explaining it to 9th-grade teachers is
more difficult.....) It must be emphasized that this uses
geometry -- not dynamics -- to define straightness.

The next step is to think some more about what it means to
have a straight line in /spacetime/. Some discussion of this,
including an interactive diagram, can be found at
http://www.av8n.com/physics/spacetime-welcome.htm#sec-straight-line-motion