On Friday the 14th Paul Camp writes about what happens if we cover up half a
lens. Perhaps this question illustrates both why students have trouble with
physics and why society places little value on learning physics. In this case
the physics teacher teaches that you block half the light. This is what
would happen if you took (for example) a simple lens, split it in half, and
inserted an opaque material through half the lens. In the real would this
is unlikely to happen and of little practical value. If on the other hand
one takes a camera lens and blocks the top half with a piece of cardboard
one does in fact block out the top half of the image. Having observed this
some smart person (who probably had not had a physics course and so had a free
mind) concluded that photographs which included land and sky could be
improved by producing filters which were half clear and half grey (or yellow,
red, etc. for b&w) so that the light from the sky would be partially blocked
and the sky would not be so washed out in the picture.
I think that the difficulty with Physics is that most real world problems
are too complex to solve with simple Physics. The problem must be broken down
into a series of simple problems, the simple problems must be worked out, and
then the results must be integrated to produce a final solution. This
discovery was Galileo's contribution to Physics. However, few of our students
have ever had any previous experience with such multistep reasoning and it is
particularly difficult in physics because the simple models, when applied
naively, often yield results which make no sense or contradict our actual
experiences.