"RangersSuck" wrote in message
On Nov 26, 9:03 am, "Ed Huntress" wrote:
"_" wrote in message
On Tue, 25 Nov 2008 19:39:56 -0500, Phil Kangas wrote:
"Doug Miller" wrote in message In article ,
"Phil
Kangas" wrote:
"Doug Miller" wrote in message In article ,
rigger
wrote:
There are only *two*. Not three. Not six. Two: The
inclined plane, and the
lever. The others are all special cases of one or
the
other of those, in that
exactly the same principle is used:
- the wheel and axle is a lever in rotary motion
- so is the pulley
- the screw is an inclined plane wrapped around a
cylinder
- the wedge is an inclined plane stood on its point
So where is the fulcrum point on a wheel and axle? A
lever
needs to
have a fulcrum point to work......
The centerline of the axle, of course.
In that case it is only the radius of the wheel. A
lever
_must_ have
three points of interest to be labeled a lever. The
point of
application,
the fulcrum point and the resultant point.
Nothing says that two (or even all three) of the points
can not be in the
same place.
The special case of a lever of zero moment -- physically
indistinguishable
from a wheel of zero radius, or a wedge of zero length. In
effect, a point.
These are good thought problems for conceptual thinking;
exercises in
practical geometry. But they're not the way to start
teaching kids how
machines work, which was the original problem.
This question of whether to teach three basic machines (or
two -- gawd)
versus six is a good thought problem itself, if the
thought it about how to
teach and how one learns. Calling a screw a "wedge wrapped
around a
cylinder" is good for geometry class or for students who
have gotten past
the ideas of how basic machines work. But it's a special
case, too, in which
any meaningful motion is rotary, whereas we think of a
wedge as something
linear. It can confuse rather than illuminate.
I learned three basic machines in physics class, too, and
it was
interesting. But six sounds better as an introduction.
Keep reducing it with
the special reductive cases and you wind up with pulleys
that do nothing and
everything else disappearing -- an interesting thought in
itself, but an
abstract one, of no use in understanding actual machines,
unless you live in
another dimension. g
I like the six machines for introducing basic mechanics.
Then show them how
these can be reduced to a smaller number. Then you can
reduce them all to a
point, at which event everything crawls up its own asshole
and becomes a
Klein Bottle in the fifth dimension. d8-)
--
Ed Huntress
But the whole idea of teaching about "simple machines" is to
demonstrate that every other machine is made of these
elemental
devices. To follow your logic, we may as well teach that a
bicycle is
a simple machine and, sometime in the future, clue the kids
in to the
fact that a bike is really a "special case" of a combination
of
wheels, levers, etc.
Sorry, I don't see how teaching a kid that there's a
difference
between a "wedge" and an "inclined plane" helps to build
their
understanding of mechanics, reason or intellect. Same with
"wheel" vs.
"pulley".
As for the Kid Down The Street, while discussing how he
might
incorporate screws into his design, I showed him some
examples in my
shop. There was the leadscrew on my lathe, the various
fasteners, and
the flutes on an auger bit. He had a hard time with the
auger, saying
something like "I don't know if my teacher will accept that
as a
screw, since it doesn't really screw anything together."
Sheesh.
I then took that auger bit and stuck it in a plastic tube
and dropped
a ball bearing in. Turning the auger, I raised the ball to
the top of
the tube. We then talked about the Archimedes Screw pump
http://en.wikipedia.org/wiki/Archimedes%27_screw and he
actually got
the idea of an inclined plane wrapped around a wheel. It's
really not
such an abstract concept.
Of course, this kid's idea of a "simple machine" is anything
that
predates an Xbox 360...
................................
Another example of lever, wheel, inclined plane would be the
good
old wheelbarrow, eih? Hey you, get these bricks up to those
masons
there, eih? Ya, just go up that there ramp, real
easy.......;)
phil