Re: [Phys-l] basic laws of motion +- vectors +- angular momentum

["Savinainen Antti" <antti.savinainen@kuopio.fi>]

Hi,

John D. wrote very interesting points regarding basic laws of motion. John offered - as always - an insightful view to physics. However, I dare to suggest another point of view to one bit mentioned by John in passing:

"... in much the same way as
 the first law is merely a corollary of the second law."

I use a text I wrote some years ago dealing with exactly this idea. Here it comes:

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Modern classical mechanics defines the first law with reference to an inertial system or inertial reference frame. Hestenes (1998, 11) defines the first law or the law of inertia in the following way:

"In an inertial system, every free particle has a constant velocity. A particle is said to be free if the total force on it vanishes."

This defines an inertial system implicitly by specifying a criterion which distinguishes it from noninertial reference frames (Hestenes 1998, 12). An inertial frame can be identified in principle by observing the motion of free particles. Since Newton’s first law is needed to define what is meant by a free particle, it cannot be viewed just as a special case of Newton’s second law. On the other hand the first law is not independent from Newton’s other laws, because they are needed to define what is meant by 'free particle'; the definition of free particle necessarily involves the concept of total force (net force) in one form or another. 

Reference: Hestenes, D. (1998). Foundations of mechanics (Originally published as Chapter 9 in the first edition of New Foundations for Classical Mechanics, 1986). Online at <http://modelingnts.la.asu.edu/pdf/Foundations.pdf>

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> From the point of view of teaching *early* introductory physics I don't mind making Newton's first law as a corollary of the second law. 

Just my two cents!

Antti


Antti Savinainen, Ph.D., B.Ed.
Adjunct Professor (University of Jyväskylä)
Senior Lecturer in Physics and Mathematics
Kuopion Lyseo High School
Finland
E-mail: <antti.savinainen@kuopio.fi>
Website: <http://kotisivu.dnainternet.net/savant/>


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