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Fig. 10.4
A cycle in the design process
From this theory a more formal dynamic model may be constructed using
mathematics. In this case a first attempt will appear as:
M1. A Body of mass M on a slide.
M2. The slide is at angle
with respect to the horizontal.
M3. The weight of the Body is W
θ
=
M*G where G is the acceleration due to
gravity.
M4. The force down the slide Fs
).
The slide is constructed and it is found that the actual acceleration is much less
than that predicted. After checking the calculations, reference is then made back to
the Theory (see Fig.
10.4
). The Theory restricts the set of possible explanations for
this discrepancy and guides the designer to create a better model. In this case, the
theory limits the set of possible proposals to the existence of another force acting
on the Body. This force then 'explains' the discrepancy between the model and the
observations; this force is called 'friction'.
The notion of friction does not come directly from the theory but is an inter-
pretation of both experience (i.e. there is a recognizable feeling of resistance when
moving the Body on the slide which becomes greater with increased pressure be-
tween the surfaces) and the Theory (i.e. only forces can influence the motion of a
Body). The process that generates the insight (that these disparate experiences and
concepts should be amalgamated into a simple causal element identified as friction)
is called 'abductive' inference. An important component of abduction is the element
of contact by an agent with the world. The Model is modified thus:
M4'. The force down the slide
=
W*Sin (
θ
Fs
=
W
∗
[Sin(q) K
∗
Cos(q)]
where K is the coefficient of friction for the materials in contact.
The coefficient of friction K is a concept that has evolved from the need to adjust
the model to fit the observations (see Fig.
10.4
). This indicates that the notion of
friction does not and cannot emerge from only the manipulation of the sentences that
make up the theory.
