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Re: Newton's 3rd Law




I have always had trouble with students misunderstanding Newton's third law.

I have lately come to realize that they don't know what a force is.

Well I'm not sure if there is a precise meaning of force, but we
may see its properties:it behaves like a vector and its effect
is to change the movement (in speed and/or direction).May be they try
to identify a 'force' in the same way as one identifies a particle.
Related with this, I would like to know if someone from the list
has experienced with:
.-Starting mechanics without kinematics, but definig
change of movement (as the effect of interacction of
particle and in the sense previously given)
.-Then introducing the term force as a suitable way
of describinf this change and showing them that it
behaves like a vector (undr composition)
.-The quantitative law would be Newton's 2nd and 3rd.
As a result, the law of inertia would be derived from the 2nd law.
I thing that when working there would be no difference.

Anyone agrees? desperate?

Are Newton's axioms realy all independent? I have never seen nothing
about this (does this signal something?), but I've some doubts.

Their only experience is the "effect" of forces. Since I have approached
Newton's 3rd from the this viewpoint they seem to understand it better.

When I start Newton's third, I stress that what they observe is NOT the
two equal forces but the effect of those forces.


Andy Jones
Physics Teacher
1600 Loop 256
Palestine Texas 75801
andjones@tenet.edu