Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: third law forces cancel?



I don't have Randy Knight's book, but it may be correct in context
because you said you are in the conservation of momentum section.

When we are looking at a single object, and we want to find the net
force on that object, we certainly want to include forces on it, but not
the reaction forces it exerts as a result of the other objects. Those
reactions belong in the free-body diagrams of the other objects.

However, when we are discussing conservation of momentum of a *system*
that contains more than one object, we distinguish between internal
forces which have canceling action/reaction pairs, versus external
forces for which the reaction force belongs in the free-body diagram of
the external agent.

Within a system, the forces between objects determine how the objects
move with respect to each other, but they do not determine how the
center-of-mass of the system moves. Action/Reaction pairs within the
system cancel with respect to finding the motion of the center-of-mass,
but they still belong in their respective free-body diagrams if you are
trying to ascertain how the objects in the system move with respect to
each other.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu