Re: [Phys-L] feeler-dealer, third law, et cetera

[Richard Tarara <rtarara@saintmarys.edu>]

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