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Solving these type of situations through free-body diagrams emphasizes
Bruce's point--there is never a weight 2 acting on weight 1 force vector
unless the objects are in contact (one above the other).
[I would suggest potato cannons at 50 meters for Bruce and John, then the
rest of us could observe Newton's Laws at work! ;-) ]
rwt
On 12/13/2013 1:59 AM, Bruce Sherwood wrote:
"By way of analogy, note that in introductory physics it is absolutely
routine
to use one weight to lift another weight with the help of massless strings
and frictionless pulleys. We speak of one weight pulling on the other
weight, and we abstract away the string that transmits the force."
It should NOT be "routine". It undercuts the important concept that
objects
in the surroundings exert forces on objects in the chosen system, either
through distance forces or through "contact" forces. I judge it to be
highly important NOT to say that one weight pulls on the other weight,
because their distant gravitational interaction is negligible, and there
is
no contact.
--
Richard Tarara
Professor of Physics
Saint Mary's College
free Physics educational software
www.saintmarys.edu/~rtarara/software.html
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