New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Teacher education: a dilemma
if physics is presented in such a way that teaching is telling and learning becomes listening, then
the subject competence of the physics teacher educator must include a special component:
the ability to simplify. Such simplification (in German ‘Elementarisierung’) means to provide
access to the ideas of physics without formalism. Only very few formalisms, and these simple,
can be used in high schools. The teacher must first use the natural language and then introduce
the concepts of physics step-by-step in accordance with the intellectual development of the
learner. Simplification requires the ability to express the idea in a language which is understandable
and meaningful to pupils. Let me give an example which has been described by the German
physics educator M. Wagenschein [ 181 .
Sixty students of a German University, all future teachers in secondary schools, had been
asked to explain the law of free fall in words. The formula s = 1/2 gt2 had been put on the
blackboard. The goal was to describe the physical content, the idea, in words and without losing
exactness, avoiding physical terms and symbols so that pupils of the appropriate age could understand
it. This is a normal task of a practising physics school teacher and should not turn out to be
difficult. Every physics teacher educator can imagine what the outcome would be were he to ask
his students this question. Probably something like: ‘The distance travelled by the falling body
is proportional to the square of the time. . . .’ But this is not a translation of the physical content
of the formula into words; this is not an idea intelligible to these pupils. What Wagenschein hoped
to hear and what the students were unable to express was something like the following: ‘Spread
two fingertips a small distance (say 5 cm) apart. Suppose this to be the distance the body falls
during the first time unit. Then the body falls during the second time unit - not the two-fold
and not the four-fold - but the three-fold distance, then in the third time unit the five-fold
distance, then the seven-fold, the nine-fold and so on.’ The odd numbers appear as Galileo had
already described. This is a ‘simplification’. It requires the teacher educator to have a deep
understanding of physics. The ability of juggling with abstract formalism is not sufficient. It does
not help them to acquire this essential teacher competence.
Another example out of my own experience: last spring, one of my physics teacher trainees
had his practice period at a senior high school. He was giving a physics class with ninth graders
(1 5 year-olds). The subject was the determination of densities of different material with different
shapes. The pupils were experimenting in groups. One group figured out that the density of wood
was 1.7, their neighbours obtained the figure 0.6. I asked which of the two was right. Both
insisted they were! I asked what density means. The answers were: ‘volume divided by mass’
and ‘mass divided by volume’. I asked again, which answer would be right. Both groups decided
not to argue but to look into the book, to ask the formula, to leave the decision to an authority.
These pupils had not grasped the idea of density. In addition they had perhaps different mental
pictures of volume and/or a wrong concept of mass. The teacher had probably never learned to
simplify, to express the idea of density in a way which made sense to pupils. Perhaps he had not
learned it because his own physics teacher found it to be far below his academic level and
incompatible with his dignity to see a problem in such a ‘simple’ thing as density. But it is
extremely important to confront future physics teachers with such exercises. In our physics
courses, students are given many exercises to do of the sort: ‘Explain in your own words the
phenomenon of buoyancy.’ ‘Assume you want to explain to a tenth grader (1 6 year-old) the term
acceleration. How would you do this?’ ‘How would you explain to a seventh grader (13 year-old)
the difference between average and instantaneous velocity?’ ‘What will a fifth grader (1 1 year-old)
probably answer if you ask for the difference between heat and temperature?’
To a physics teacher trainee who has only 6 or 8 semester periods at the university for picking
up his professional competence it is more important to spend time in such exercises than to try,
in nuclear physics, to prove mathematically that for a spherically symmetric charge distribution
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