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[Phys-L] Re: A Third law question



The student's question goes deeper when you consider situations which do not
involve deformation as the principle model of the reaction force. You can
develop a much larger force when throwing a baseball than when throwing a
ping pong ball - in neither case is deformation of the thrown object a
significant factor. This comparison brings to the fore the "ma" inertial
force. You can control your arm's acceleration to a large degree, but the
force developed cannot exceed that acceleration times the mass of the thrown
object.

Since we have chosen "ma" as the measure (definition) of "force", N3 must
hold in a 2 body interaction, else a body composed of two interacting parts
could self accelerate (which is not what we observe).

In a larger context, conservation of linear momentum forces our "ma"
definition of "force" into an assertion of N3 for a 2 body interaction.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.winbeam.com/~trebor/
trebor@winbeam.com|