Re: F=ma

["Miguel A. Santos" <msantos@ETSE.URV.ES>]

On Wed, 10 Nov 1999, Leigh Palmer wrote:

> At 01:45 -0800 11/10/99, Miguel A. Santos wrote:
> > On Tue, 9 Nov 1999, Leigh Palmer wrote:
> >
> > > I recognize that what I've said above applies also to Newton's
> > > second law when it is applied to forces of contact. There is a
> > > possibility of a causal element when Newton's law is applied to
> > > forces that act at a distance, however.
> >
> > But not within Newtonian mechanichs, isn't it?
> > Could you explain a bit more what you mean in this last sentence?
> > I suppose you're thinking in terms of an interaction which travels with
> > finite velocity. But for Newton, altough he admitted no to be comfortable
> > with it, the forces acted intstantly. It would seem that not even for
> > forces acting at a distance could you mantain then your 'causal element'.
>
> Yes, that's what I had in mind. While the Natural speed limit was
> not part of Newton's phyics, it is somehow part of the package that

Hence, can one say: no causal element at all there, just in our mind?

> comes to us as a beginning student. It is in the popular culture,
> even if it got there by nefarious (non-physics teaching) means.

 Not sure about that. Consider the following question posed to a HS
student or to one of first semester of a university studies (engineery,
physics,chemistry,...): If charge Q is moved 1cm away from its original
position in 1 second, how long does it take to another charge q to notice
this change if it is d=10^6Km away from Q?
        Altough all may know that *nothing* moves faster than
light, do you expect all to give the correct answer (d/c)? even at least
something above 1s.?
        I don't think so. Their notion about the limit of c can be really
vague. I wouldn't expect it as already given.


>  > Well, I'm a physicist and I got time to learn this point, but what about
> > the three-years ingeneers studies? Here in Spain, they all have an almost
> > common physics course during his first semester. Should one also care
> > about how to state things like F=ma? Is it worth enough, I mean, does this
> > care improves really their understanding?
> > What about at High School? Is this refinement really effective or can it
> > be subtituted by letting them do enough exercises, discussion,...?
>
> I recognize no fundamental differences among groups of students
> classified by discipline*. There are probably larger differences in

 The point was not that much in the discipline, but the orientation of it:
3 years is different than 5 years. Here, companies expect also
differents things of each type engineer, the first one being not supposed
to reach a directive position as high as the second one. This last one
also gets a more firm basis of the fundamentals of the science s/he
studies.
 Another great difference here is the entrance selection. Altough also
based on their grades, there are two pathways to get to the university:
coming from the HS, or from another way (similar to the german
'Berufsshule'), more profession-oriented. Both have different backgrounds,
but are taught together. The math background of the second group is quite
lower than that of second.

> classifying them by sex, and those differences are not great. The
> recognition of a much greater diversity of ability to assimilate
> new concepts within any particular group is one I have to keep
> reminding myself of repeatedly.

 That is a good point. Thanks for sharing your experience with me (us).

 So I understand that you think it IS worth indeed to worry about how to
present them 'things like F=ma'. The point is to improve the odds; the
brilliant students might also learn doing a lot of exercises.


 Best regards,
        Miguel A. Santos
        msantos@etse.urv.es