Re: [Phys-l] the first law of motion

[John Denker <jsd@av8n.com>]

On 11/01/2011 12:50 PM, chuck britton wrote:
> Every time I hear the current tv ad that proclaims that a body in 
> motion TENDS to stay in motion I have to refrain from yelling
>
>   "WILL stay in motion".

Since I am the only one who has recently used the expression
"tends to remain at rest" in this forum, I suppose I should 
say something about this.

The expression "tends to" is not meant to be precise.  Those
words practically scream a warning that parts of the story are
being left out.

I think we all agree that precise statements are preferable to
imprecise statements.

OTOH I have never worried too much about stating the first law
precisely, because for most purposes it suffices to state the
/second/ law precisely, and treat the first law as a special
case of the second law, namely the F=0 case.

> To me it seems to be a bit of a bone thrown to Aristotelian thinking.

That was definitely not my intent.  To the extent that I thought
about it at all, to the extent that I intended anything at all,
my intent was
 a) to stick to the standard, colloquial, colorful statement of
  the law, and 
 b) to say something that was obviously not meant to be precise.

Neither of those can be considered good reasons or even good
excuses.  I'm not recommending the "tends to" language.

>   "WILL stay in motion".

That seems to go a step too far in the opposite direction.  If
you want a precise statement, you have to work harder than that.

My second-best recommendation is to try some wording like this:
  An object in motion will remain in motion, namely uniform 
  straight-line motion ... *provided* there are are no outside 
  forces acting on it.

My actual best recommendation is what I said before:  For most
practical purposes the best approach is to state the /second/ 
law carefully.  The first law is then a minor corollary.

The first law has huge historical significance, but that's quite
a separate discussion.  There is no law that says pedagogy must
recapitulate phylogeny.  That is, retracing history is rarely the
best way to teach a subject or to solve practical problems.