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Using Blocks to learn Fractions (3 part essay)
Posted:
Mar 1, 2010 4:11 PM
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DISCOVERING VOLUME
Those of us familiar with the Montessori design for a pre-school,
featuring off-the-shelf "jobs" with which the children discover
(both alone and in groups) know that "playing with blocks" is an
important aspect of this curriculum.
When it comes to Fractions, one may use various senses of
"conservation" to demonstrate equivalence. Pouring water or
dry granular material from one container to another is a good
way to demonstrate relative volume: four cups make a quart,
four quarts make a gallon, eight ounces make a cup.
DISTINGUISHING VOLUME FROM WEIGHT
There's a blend of weights and volumes you'll notice, because
once the density is set, and presumably the substance, you're
able to use volumes to compare weights and vice versa. Twice
as much of something is both twice the volume and twice the
weight.
More formally, however, we distinguish between volume
and weight as different "dimensions" and start to tease out
these differences in discussions of physical principles. The
same volumes will have different weights on the moon i.e.
the gravitational field is a variable.
RES EXTENSA
Decartes and his contemporaries referred to pure extension,
minus any secondary characteristics of temperature, weight,
color, density, as Res Extensa. The XYZ coordinate system
is its own best example, in spanning a volume without
contributing any mass or material of its own. The XYZ system
is a "reference frame" meaning we impose it as an imaginary
ruler or grid. Even when we construct our rulers out of physical
materials (as we often do), we keep this sense of the "purely
imaginary" and refer to that vista when doing "solid" and/or
"spatial" geometry.
I think many students start having philosophical confusions
right about here. Many teachers don't have the philosophical
training to tease apart the various nuances and a student
may lose track of how a "dimensionless point" differs from a
"dimensionless cube", as neither one has any physical
existence to speak of (conflating two meanings of
"dimensionless").
In spatial geometry, we may eventually want to model the
weight of something, in which case it turns into physics
and/or trading in the market place (weights and measures).
What's important to get is this isn't the starting position
i.e. the XYZ coordinate system is not a physical scaffolding
and therefore has no physical dimensions as these might
relate to time and/or space (space-time) in the sense of
having energetic content. We speak of energy-based
mathematics as "applied" and/or "about the real world".
The no-energy world of res extensa is frequently referred
to as "Platonic".
FRACTIONS AND POLYHEDRA
We've been discussing here on math-teach about how the
linear, areal and volumetric rates of change, of a shape with
fixed angles, are related as first, second and third power
quantities respectively.
Starting with a tetrahedron of volume one, and doubling all
its edges, results in a tetrahedron of relative volume eight.
Halving the edges from the same starting position gives
a tetrahedron of relative volume 1/8.
Surface area varies as a power of two, as we see in the
many formulas using exponential notation (i.e. most all
of them).
Most 1900s textbooks, as well as most early Montessori
jobs, introduced these concepts almost exclusively with
little cubes as units. A rod of cubes would represent an
edge or line. An n x n square of cubes represented a flat
surface (growing as a 2nd power of n), and the 3rd power,
or volume, was made of stacked squares. n x n x n was
modeled as a cube with sides n.
This modeling technique isn't going to fade away, but nor
is it sacrosanct. Mathematics is a flexible discipline, and
what better way to illustrate this fact than to switch bases?
Just as we go back and forth between decimal (10) and
hexadecimal (16), so might we switch between triangular,
60-degree based thinking, and the more orthodox 90-degree
based thinking. As a mnemonic, we note that 10 is a
triangular number, whereas 16 is a square one. Mathematics
allows this kind of diversity. Sometimes (often) two ways
of looking is better than one.
One shortcoming of cubes is they don't play with others
that well, only with themselves. The volume formulas for
almost all polyhedra generate irrational numbers, because
little thought was given to a more integrated design.
Another problem with cubes is they're topologically more
complicated than tetrahedra, being hexahedra.
One might easily imagine a tropical island paradise where
the natives pack together three coconuts (as idealized
equal-radius spheres) and call that a unit of area, pack
together four coconuts (as a tetrahedron) and call that
a unit of volume. Why not study this island culture then,
explore the consequences of this more primitive topological
beginning? Might this civilization turn out to be more
advanced than our own? Or perhaps we should look at
Martian Math? Here in Oregon, we're looking at both
of these storyboards as useful backdrops for future
mathcasts (animations, cartoons).
Cubes do have the advantage of filling space however, thereby
generating our imaginary XYZ coordinate system (once you
pick an origin and basis edges), so of course we don't want
to lose them or abandon centuries of useful algorithms (that'd
be wasteful). Cubes are cool, but not the whole story, is what
these segments on Fractions will make clear.
The NCTM lesson plan on tetrahedral kites goes through the
logic needed to prove how the regular octahedron of edges N,
has 4x the volume of the regular tetrahedron of edges N. We
start talking about V + F = E + 2 at this point, building these
shapes from ping pong balls, and spinning them around their
various axes -- all branches to other curriculum topics. The
kite itself has an historical dimension connecting us back to
Alexander Graham Bell, as most math teachers will know, if
aware of this lesson plan in the first place.
Using the regular tetrahedron and octahedron together, we
are able to fill space (same as with XYZ). We have a
relative population of 2:1 (twice as many tetrahedra) and
a volume ratio of 1:4 (per tetrahedral kites). That's an
excellent beginning for discussing fractional relationships,
but where is our cube? A regular tetrahedron embedded
as face diagonals in a cube has one third its volume, another
math fact easy to prove visually, as well as algebraically.
Or just pour water or dry grain if doing a Montessori job
(a self directed activity). The cube contains the unit
tetrahedron and has a relative volume of three.
RHOMBIC DODECAHEDRON
This shape gets special attention in any spatially
aware geometry curriculum. Kepler gets a lot of
credit for "tuning it in" (a radio metaphor). Given it's
neither a Platonic nor an Archimedean polyhedron,
it was languishing in obscurity, not appreciated for
its marvelous properties.
What are these marvelous properties?
Well, for one thing, it fills space completely, without gaps.
You can put a sphere inside each one, and they'll just
"kiss" one another at the diamond face centers. That's
twelve spheres around any nuclear sphere. Do we have
the models in class? If not, shall we make them? That's
what Linus Pauling did, and he won two Nobel Prizes,
one for chemistry.
For another thing, the long face diagonals define an
octahedron, and the short face diagonals define a cube.
These two shapes are "dual" to one another, a concept
we explore.
Taking both of these (the cube and octahedron) to be the
same as those described above (volumes 3 and 4
respectively), and we have a great set of geometry blocks
to play with: a tetrahedron and octahedron that fill space
together, and a rhombic dodecahedron that fills space by
itself, for example by surrounding each corner where the
first two share a vertex.
In sum, here we have two space-filling matrices or lattices,
interpenetrating, the one of two complementary shapes
(tetrahedron + octahedron), the other of all the same shape
(rhombic dodecahedron).
That's a lot to get across in words, but on a bright, sharp
computer screen, you're looking at enlightening cartoons
(animations, mathcasts). You may also build these as
hands-on models, as is done in the NCTM lesson plan
on tetrahedral kites.
Note that "kites" is the visual motif for the next NCTM
annual session. How might we relate kites to math topics?
In many ways, with tetrahedral kites being one of them
(Penrose kites is another, with links to Kepler again).
VOLUME RATIOS
Our 2000s curriculum is no longer restricted to only sharing
about cubes when it comes to describing space and the
relative volumes of polyhedra. Per the NCTM lesson plan
on tetrahedral kites, we have the option to build with a
different set and some curricula, including mine (pioneering
in Oregon), take advantage of this option.
There's no presumption of either/or, and in showing how
our regular tetrahedron might be used to calibrate this
alternative set of blocks (volumes 1, 3, 4, 6 for tetrahedron,
cube, octahedron, rhombic dodecahedron respectively),
we're not letting go of the more traditional cubical mensuration
system.
The fractions 1/3, 1/4, 1/6, 3/4, 1/2 derive easily from this
new set of blocks. As we spiral through in future segments,
we will add to this vocabulary, both by splintering the regular
tetrahedron and octahedron into smaller tetrahedra of volume
1/24, and by adding additional polyhedra, including some
from the five-fold symmetric family, such as the rhombic
triacontahedron, icosahedron, and pentagonal dodecahedron
These latter three have a relationship that parallels that of the
rhombic dodecahedron to its embedded shapes, as defined
by its rhombic facets. Our rhombic triacontahedron of volume
5 splinters into 120 tetrahedral modules equal to 1/24.
SPIRALING
Sometimes teachers question whether doing the work to
introduce these blocks is going to grow with the student,
or immediately become back burner and forgotten as one
of these elementary school topics that's likely a bridge to
nowhere.
Lets reassure these teachers that rhombic dodecahedra in
closest packing is likewise chemistry. The octahedron-
tetrahedron lattice is likewise architecture. Those branching
topics mentioned earlier, derived from spinning, packing with
ping pong balls, looking at V + F = E + 2 and the concept
of dual, are all gateways to a huge and multi-disciplinary
literature.
Not only that, but spatial geometry includes all the flat lander
stuff as a special case. The trigonometry of flat surfaces, as
well as curved ones, is embedded in this context, so none
of the Euclidean material leaks away.
So the real question is whether spatial geometry as a whole
is relevant, as once we admit that it is, then we're somewhat
obliged to acknowledge its core content and would be foolish
to bleep over its most streamlined encapsulations.
Spatial geometry underwent a lot of changes in the 1900s
and just sticking with the awkward volume formulas and
those mostly irrational number treatments (except where
the cube is concerned) is a sure way to fall behind, relative
to more enlightened approaches, already available.
The more 60-degree based way of thinking described above
is consistent with findings about naturally occurring geometries,
which take advantage of the triangle's (tetrahedron's) superior
stability vis-a-vis the square's (cube's). The emergence of
corresponding mathematical treatments has been a long
term trend. One could list many published titles at this point,
such as 'Beyond the Cube'. Dr. Arthur Loeb of Harvard and
MIT helped get the ball rolling in 1966, in some issue of
Math Teacher. None of this is new information at this point.
Its phase-in is long overdue and above professional reproach,
given its secure mathematical basis.
LEVERAGE FOR TEACHERS
Teachers chafing under the restrictive nature of state standards
might want to assert their professional status and call into
question the authority of any standards body that is still
mired in 1900s cubes-only thinking. Even at the elementary
school level, such "rule by committee" is manifestly without
integrity and is doing much to hold back innovation and
academic success.
Of course it may not be wise to question authority that overtly.
A teacher may prefer to quietly phase in this material, using
some of the lesson plans already on-line, just to establish a
track record for future review. It's always nice, in hindsight,
to be seen as an "early adopter" in some relative sense, even
though in a conservative discipline such as mathematics,
upgrading is often a slow process.
Our K-12 curriculum is something of a relic as early math
mostly changes in geological time, although a punctuated
equilibrium model may pertain.
Even just a dash of this more contemporary thinking goes
a long way, like a spicy hot sauce, so it's OK to use it
sparingly, experimentally. Find your peers and collaborate
on a "place based" approach. Don't wait for a wood pulp
textbook to spell it all out. Those dino days are likely over.
To *not* use the more sophisticated blocks in any way, shape
or form is to settle for bland "business as usual" a somewhat
risky (and vapid) choice in today's climate, if hoping to seem
in any way current (up to date). Let students vote with their feet?
At least project some of the Internet material and see if they
perk up a little. In my experience, they usually do, especially
when they pick up on the subversive flavor, of challenging
some crufty-dusty status quo -- there's the scent of a hunt,
a game, a sport.
Mass-published elementary school textbooks of course have
nothing like this, high school textbooks precious little, so
they become exhibits in our "museum of retro". It's fun
taking these apart in class, a kind of archeology, and pointing
out the strong cubic bias -- a lot like investigating racism,
in terms of finding the tell tale signs.
Remember: all mathematics is ethno-mathematics.
I would encourage teachers to engage student interest at
this level, as "critical thinking" applies not just to what
you see on television, but what you read in the paper and
find in your textbooks. Unthinking acceptance of media,
of mathematics included, is the antithesis of developing
those critical thinking skills we seek to encourage (those
of us in the liberal arts tradition, who value philosophy and
its freedoms -- not saying everyone does).
(to be continued)
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