Re: [Phys-L] treating force as a vector ... consistently

[Moses Fayngold <moshfarlan@yahoo.com>]

On Saturday, August 20, 2016 8:45 PM, Todd Pedlar <pedlto01@luther.edu> wrote:


 
> As long as I never refer to actions or reactions (I actively oppose this
> the first day we talk about forces in my courses) ...
If only the first day, it is OK. But if permanently, then why?
> ...I specify the following:

> 1) The two forces which constitute a Newton's Third Law Pair must act on
> different bodies
> 2) They must be of the same kind (friction, gravitation, contact forces, etc)
> 3) When considering the state of motion of a body, one must consider all
> the forces on (and only the forces ON) that body by any other agent
This is an excellent clarification which is frequently lacking in teaching. When a book is resting on a table, there is downward gravitational force (action) on the book from the Earth and the upward gravitational force (reaction) on the Earth from the book. We have a pair of action-reaction forces of the same kind applied to two different bodies. 
 In addition, there is an upward normal force (another action) on the book from the table and the downward normal force (the respective reaction) on the table from the book. Altogether, we have two distinct pairs of action-reactions: one gravitational between book and the Earth, and one normal (contact) between the book and the table. Unfortunately, students often confuse (downward gravitational force on the book + upward normal force on it) with the action-reaction pair. Formally, they are equal and opposite, but they are 1)applied to the same body and 2)are of different kind, which takes them out of "action-reaction" category. One way to clarify the confusion would be, e.g., to push the table upward with some acceleration. Then the normal force on the book, while remaining opposite to the gravity force, would exceed it in magnitude. This shows that they are not action-reaction pair.   But we must also warn the students, at least at some later stage, that the initial simple definition of "action-reaction" works only in the static classical mechanics. Already in Electrodynamics with 2 point charges in relative motion the forces between them are generally neither opposite nor equal in magnitude. In such cases a better description of reality would be momentum of particles and their fields. 
Moses Fayngold,NJIT

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