[Phys-L] Re: A Third law question

[hhaskell at MINDSPRING.COM (Hugh Haskell)]

At 15:25  -0500 10/11/05, Rick Tarara wrote:
> These are both great suggestions for a student in my science majors class,
> but neither are going to work well with students from my gen-ed class.  I
> talked to Joe Bellina here at length about this, and we sort of came to the
> conclusion that one pretty much has to take Newton's laws as axiomatic and
> then look to their applications as 'proof'.  Most of my attempts were trying
> to use 2nd law ideas to try and get at the 3rd law, and that can't be done.
> This student is bright and WILL accept that N3 is true but is trying to
> understand HOW it could be true in a situation that she feels is
> contradictory.  She wants a mechanism like the lattice one above, but is
> most likely not knowledgeable enough to accept that particular argument.
Let me reiterate what John D. said. Conservation of momentum is the
best way I know of to get students to understand forces and their
affects, including N3. It doesn't require fancy equipment to get
students to accept it, only a few demonstrations and some hands-on
work by the students. If these are gen-ed students, they really need
to get that hands-on work, so they will know what to do with their
students in due course, and why.

As to using deformation to generate the force idea, it can also be
relatively easily demonstrated without fancy equipment, by using a
form of optical amplifier. anchor a rod to a wall with some modeling
clay, then rest the rod on a block with a dissecting needle
"borrowed" from biology between the rod and the block. Glue a bit of
mirror onto the needle, using the handle end of the needle as a
support stage for it, and then shine a laser into the mirror so that
it reflects onto the opposite wall. When someone leans on the wall,
and it inevitably moves a few microns, the rotation of the needle
will allow the laser spot to move a large amount, easily seen by the
whole class. It is very graphic, and it makes it easier for them to
understand that "solid" objects are not inflexible, thus making it
easier for them to accept that that iron block can be deformed and
that can give rise to forces.

Hugh
--

Hugh Haskell
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<mailto:hhaskell@mindspring.com>

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