The Nature of Physics
The Nature of Physics
The study of physics is a tribute to the human intellect and its use of
knowledge and reason to invent mathematical models of reality.
Physics is one of the liberal arts, combining elements of reason,
philosophy, mathematics, language, and rhetoric. Historically, physics
was called natural philosophy; we gave up much nuance when this
phrase was abandoned during the 19th century.
It is in this spirit that we, as students and professors alike, study nature,
constantly observing, changing, and modeling the world around us with
all the real and theoretical tools that we, as an intelligent species, can
invent. All of us are natural philosophers; we all are physicists.
The term natural philosophy reflects the creative and dynamic interplay
that exists in physics among experiment, theory, logic, insight,
inspiration, symmetry, beauty, and language.
Descriptions of Nature, But Not Nature
Kinematics, a quantitative description of motion, was invented by
Galileo Galilei in the early 1600s after much observation and
experimentation. We say invented rather than discovered, since
physical theories do not lie lurking in nature, waiting to be discovered
like an unknown planet or plant. Rather physical theories are
inventions of the human intellect that describe and account for
observations of nature.
Forcetotal = (mass) x (acceleration)
The expression ma is not a force, although it has the same dimensions
as a force (because both sides of an equation in physics must have the
same dimensions). The quantity ma is equal to the magnitude of the
total force, but is not itself a force.
Dimensions
Dimensions
Length
One of the “meanings” of the equals sign
Time
Mass
Charge
Dimensions are the fundamental quantities needed to
converse quantitatively ( and for that matter qualitatively)
about the physical universe.
Dimensions have units; they are not units.
The word dimension has two different meanings in physics. One
meaning is geometrical: a dimension is any of the least number of
coordinates needed to specify the location of a point in space.
The second meaning of the word dimension is quite different. We say
the dimensions of the density of a material are mass per unit volume.
Likewise, the dimensions of speed are those of distance divided by a
time interval. The measure of density and speed depend on the unit
system used.
Dimensional Analysis
For physical equations to be correct they must be dimensionally correct.
Example:
E = mc2
"Heavier than air flying machines are impossible.”
Could it be E = mc3 ?
(Lord Kelvin, president, Royal Society, 1895)
E = energy therefore has MKS units of Joules = N m = kg m2 /s2
Dimensionally this is mass length2 / seconds2
The right hand side of the expression must have the same dimensions
therefore it must be mc2 and not mc3.
"I think there is a world market for maybe five computers."
(Thomas Watson, chairman of IBM, 1943)
"There is no reason for any individual to have a computer in their home."
(Ken Olsen, president, chairman and founder of Digital
Equipment Corp., 1977)
"Airplanes are interesting toys but of no military value."
(Marshal Ferdinand Foch, French commander of Allied forces
during the closing months of World War I, 1918)
"The telephone has too many shortcomings to be seriously
considered as a means of communication. The device is inherently
of no value to us.”
(Western Union internal memo, 1876)
"The wireless music box has no imaginable commercial value.
Who would pay for a message sent to nobody in particular?"
"Who the hell wants to hear actors talk?"
(Harry M. Warner, Warner Brothers, 1927)
(David Sarnoff's associates, in response to his urging for
investment in radio in the 1920's)
"Everything that can be invented has been invented."
(Charles H. Duell, commissioner, US Office of Patents, 1899)