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Re: momentum before force



However, one should be careful about thinking that the ancient concept of
impetus is equivalent to our definition of momentum; it is not. The earlier
concepts of Impetus seem to encompass both kinetic energy and momentum in
some odd way. An example of possible confusion in terms can be found in a
1949 translation of Galileo by Henry Crew and Alfonso de Salvio, Galileo
discussed (in a section on Acceleration and Laws of Falling Bodies) a lead
bullet hanging from a string attached to a nail in a wall. The bullet is
alowed to swing like a pendulum from a string so that the string strikes a
second nail in the wall below the first. Galileo discusses the height that
the bullet rises to after the string strikes the second nail and gives an
argument based on what is termed "momentum" in this translation. Our
persent day interpretation would include a discussion of kinetic energy
rather than momentum. The definition of impetus is very murky and does not
correspond exactly to our definition of either momentum or kinetic energy.

Also Aristotle gave a definition of impetus as a force which pushed
something forward. In his reasoning as to why an arrow moves (since he
insisted that natural motion of objects cause them to come to a halt) was
that air rushes in behind the arrow as it travels, pushing it forward.

kyle

Paul: as you say, the idea of momentum has an ancient history in which is
co-evolved with the idea of force. I'm looking for a way to short-circuit
that evolution by motivating momentum *without* the idea of force ---
purely so that only one new concept needs to be introduced at a time.

--James McLean

In the history of mechanics, Oresme and Buridan developed the idea of momentum
as the natural "quantity of motion" -- a quantity proportional to how much
stuff is moving and how fast it moves. Nicole Oresme worked around 1350
(250 years before Galileo, who also worked on the same idea before Newton).
Jean Buridan (50 years before as rector of the university of Paris) developed
the roots of Oresme's work on what was then called IMPETUS. Galileo and Newton
followed both.

Newton worked in dP/dt -- the change in momentum (he was too cagey to
assume constant mass like the statement F = ma is often misinterpreted).
From IMPETUS or the better-differentiated MOMENTUM that followed, he
defined force.

There are web entries on all these people through the DEC ALTA VISTA engine,
and both _The Mechanical Universe_ and Piaget & Garcia's _Psychogenesis and the
History of Science_ discuss this. Alternatively, one could take EDCI 695
here at Purdue and learn about these ideas and many other pertainent insights
into how human beings learn and do physics as part of a graduate degree
in science education. We have a number of graduate physics students doing
so voluntarily and finding the experience to be worthwhile.

Dan M

Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ Univ
Visiting Asst Prof, Purdue Univ Physics; Adjunct Faculty, Indiana U at Kokomo
NEW NET ADDRESSES: danmac@nau.edu http://www.phy.nau.edu/~danmac
Ph.D. in Education: Curriculum & Instruction

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