休 D.楊（Hugh D.Young） is Emeritus Professor of Physics at
Carnegie Mellon Universityin Pittsburgh，PA.He attended Carnegie
Mellon for both undergraduate and grad-uate study and earned his
Ph.D.in fundamental particle theory under the direction 0f the late
Richard Cutkosky.He joined the faculty of Carnegie Mellon in 1956
and has also spent two years as a Visiting Professor at the
University of Califomia at Berkeley. Pro Young’S career has
centered entirely around undergraduate education.He has written
several undergraduate-level textbooks，and in 1 973 he became
aCO-authorwithFrancis Sears
andMarkZemanskyfortheirwell-knownintroduc-tory texts.witIl their
deaths，he assumed full responsibility for new editions 0fthese
books until joined by Prof.Freedman for Uiversity Physics.
Prof.Young is an enthusiastic skier,climber,and hiker.He also
served for sev.eral years as Associate Organist at St.Paul’S
Cathedral in Pittsburgh，and hasplayed numerous organ recitals in
the Pittsburgh area.Prof.Young and his wife Alice usually travel
extensively in the summer,especially in Europe and in the desert
canyon counthern of southern Utah.

The development of physical theory requires creativity at
every stage.The physicist has to learn to ask appropriate
questions，design experiments to try to answer the questions，and
draw appropriate conclusions from the results.Fig-ure 1～shows two
famous experimental facilities. Legend has it that Galileo
Galilei（1564-1642）dropped fight and heavy objects from the top
ofthe Leaning Tower ofPisa（Fig.1.1a）to find out whether meir rates
of fall were the same or different.Galileo recognized that only
experi.mental investigation could answer this question.From
examining the results of his experiments（which were actually much
more sophisticated than in the leg.end），he made the inductive leap
to the principle，or theory，that the acceleration of a falling body
is independent of its weight.

The development of physical theories such as Galileo’s is alwaysa
two.way process that starts and ends with observations or
expedmen~.This development often takes an indirect path，with blind
alleys，wrong guesses，and the discarding of unsuccessful theories in
favor of more promising Ones.-Physics is not simply a collection of
facts and princil；}les；it is also the process by which we arrive at
gen.eral principles that describe how the physical universe
behaves. No theory is ever regarded as the final or ultimate
truth.The possibility always”exists that new observations will
require that a theory be revised or discarded.It isin t11e nature
of physical theory that we can disprove a theory by finding
behaviormat 1s inconsistent wlth 1t，but we can never prove that a
theory is always correct. Getting back to Galileo，suppose we drop a
feather and a cannonball.Thevcertainly do not faU at the same
rate.This does not mean that Galileo was wrong；lt means that his
theory was incomplete.If we drop the feather and the cannon.ball in
a Vacuum to eliminate the effects ofthe air,then they do fall at
the samerate.Galileo’s theory has a range of validity：It applies
only to objects for whichthe force exerted by the air（due to air
resistance and buoyancy）is much 1ess thanthe weight.Objects like
feathers or parachutes are clearly outside this range. Every
physical theory has a range of validity outside of which it is not
appli，cable.Often a new development in physics extends a
principle’s range of valid-ity.Galileo’s analysis of falling bodies
was greatly extended half a century laterby Newton’s laws of motion
and law of gravitation.

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