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

[Philip Keller <pkeller@holmdelschools.org>]

In fact, even when they are in contact, you can get in trouble saying that
weight2 "acts" on weight1.  Better to make the separate free-body diagrams
with "normal forces" (or "perpendicular surface forces" as you prefer).
 I'm thinking of a 5 kg block sitting on a 10 kg block sitting on a
platform that is accelerating upward at say 2 m/s2...


On Fri, Dec 13, 2013 at 9:10 AM, Richard Tarara <rtarara@saintmarys.edu>wrote:

> 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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