"_" 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