3B Scientific Air Cushion Plate - Handbuch (Seite 56)

5
Physical Experiments on the Air-Cushion Table
Introduction
Air cushions are produced and sustained by
means of air continuously emitted from jets in
one of the objects as they move against one an-
other. This prevents any contact between the two
objects. As a “lubricant”, there is a thin gas cush-
ion between them, similar to the oil film frequent-
ly used. Due to the much lower viscosity of the
air, friction is reduced to negligible levels.
Using the air cushion makes it possible to con-
duct many experiments in a much better quality.
A large number of experiments, however, are only
possible by making use of the air cushion.
A disadvantage of the common two-dimensional
air-cushion arrangement is limited visibility. To
observe the motions in two dimensions, it is nec-
essary to step up closely to the setup. Such sys-
tems are furthermore very difficult to handle be-
cause of complicated stabilization and adjustment
procedures. The use of projection offers new
opportunities. It allows both an expedient reduc-
tion in the size of all parts of the experiment set-
up and a considerable improvement in visibility.
Finally, mechanical collisions proved to be too
inefficient. Since in this case only part of the en-
ergy is transmitted, it would have been neces-
sary to take additional measures to compensate
for the loss of kinetic energy. Making use of the
forces between ceramic magnets allows the pro-
duction of virtually fully elastic collisions. The
fact that an immediate contact between the col-
liding partners does not occur is no disadvantage
in most cases. This method is highly suited for
model demonstrations, e.g. of the force relations
on a microphysical level.
The gas cushion principle, use of projection and
use of magnetic forces make the air-cushion ta-
ble a high-quality teaching aid, characterized by
simple operation, high reliability, universal us-
age and excellent methodological qualities. Some
of the experiments basically cannot be carried out
better with other currently known methods.
The air-cushion table is used mainly in model
demonstrations of microphysical procedures. The
characteristic vividness of models and the excel-
lent visibility make this demonstration a kind of
“window into the microcosm”. However, it is nec-
essary to mind the shortcomings and limits of
modeling. Not only are the procedures highly sim-
plified and represented in a purely mechanical
way, also the motions of the real objects are in
many cases determined by other forces. Further-
more, all procedures occur on one level. Finally,
models contain additional misrepresentations,
which become visible e.g. in the shape and color
of the hover discs.
Due to the relatively high throughput of air and
the small size of the hover discs additional driv-
ing mechanisms occur. The effect of these is that
the motion of the small, hover discs will not stop
as long as the airflow continues. This has the great
didactic benefit that many processes can be ob-
served for any required duration, without any
need of intervention.
On the other hand, the limited force effect be-
tween the hover discs and between them and the
magnetic barriers determines a specific maximum
speed, which, in the case of very quick hover
discs, has practically been reached already after
one collision.
With the help of these mechanisms, optimal, well-
visible motions usually begin by themselves. The
driving mechanism increases the velocity; the not
fully elastic collisions limit it.
However, both mechanisms can also have an ad-
verse effect by misrepresenting the motions of
interest. Only the knowledge of these processes
and their well directed usage or inclusion by the
experimenter allow full utilization of the great
potential of this valuable teaching aid.
On the following pages you will find a descrip-
tion of the setup and possible uses of the air-cush-
ion table. Then you will find instructions for con-
ducting important experiments.
The illustrations are meant to assist you in your
work. They are taken from the perspective from
which the teacher views the experiment setup on
the air-cushion table.