1.0 - Introduction
Heavyweight, lightweight, overweight, slender. Small,
tall, vertically impaired, “how’s the weather up there?”
Gifted, average, 700 math/600 verbal, rocket
scientist. Gorgeous, handsome, hunk, babe.
Humans like to measure things. Whether it is our
body size, height, IQ or looks, everything seems to be
fair game.
Physics will teach you to measure even more things.
For example, quantities such as displacement,
velocity and acceleration are crucial to understanding
motion. Other topics have yet more things to quantify:
Mass and period are concepts required to understand the movement of planets; resistance and current are used for analyzing electric circuits.
Just as you have developed a vocabulary for the things you measure, so have physicists.
There are many different units for measuring different properties. It is possible to go all the way from A through Z in units: amperes, bars,
centimeters, dynes, ergs, farads, grams, hertz, inches, joules, kilograms, liters, meters, newtons, ohms, pascals, quintals, rydbergs, slugs,
teslas, unit magnetic poles, volts, webers, x units, years, and zettabars. (OK, we had to stretch for X, but it is a real unit.)
Physicists have so many units of measure at their disposal because they have plenty to measure. Physicists use amperes to tell how much
electric current flows through a wire, “pascals” quantify pressure, and “teslas” are used to measure the strength of a magnetic field. If you so
desired, you could become a units expert and impress (or worry) your classmates by casually noting that the U.S. tablespoon equals 1.
Canadian tablespoons, or deftly differentiating between the barrel, U.K. Wine, versus the barrel, U.S. federal spirits, or the barrel, U.S. federal,
all of which define slightly different volumes. Or you could become an international sophisticate, telling friends that one Germandoppelzentner
equals about 77,162 U.K. scruples, which of course equals approximately 101.97 metric glugs, which comes out to3120 ukies, a Libyan unit
used for the sole purpose of measuring ostrich feathers and wool.
Fortunately, you do not need to learn units such as the ones mentioned immediately above, and you will learn the others over time. Textbooks
like this one provide tables that specify the relationships between commonly used units and you will use these tables to convert between units.
1.1 - The metric system and the Système International d’Unités
Metric system: The dominant system of
measurement in science and the world.
Historically, people chose units of measure related to everyday life (the “foot” is one
example). Scientists continued this tradition, developing units such as “horsepower” to
measure power.
The French challenged this philosophy of measurement during their Revolution, when
they decided to give measurement a more scientific foundation. Instead of basing their
system on things that change í the length of a person’s foot changes during her
lifetime, for example í the French based their system on what they viewed as constant.
To accomplish this, they created units such as the meter, which they defined as a
certain fraction of the Earth’s circumference. (To be specific: one ten-millionth of the
meridian passing through Paris from the equator to the North Pole. It turns out that the
distance from the equator to the North Pole does vary, but the metric system’s intent of
consistency and measurability was exactly on target.)
The metric system is also based on another inspired idea: units of measurement should
be based on powers of 10. This differs from the British system, which provides more variety: 12 inches in a foot, 5280 feet to a mile and so
forth.
The metric system makes conversions much simpler to perform. For example, in order to calculate the number of inches in a mile, you would
typically multiply by 5280 (for feet in a mile) and then by 12 (for inches in a foot). However, in the metric system, to convert between units, you
typically multiply by a power of 10. For instance, to convert from kilometers to meters, you multiply by 1000. The prefix “kilo” means 1000.
The revolutionaries were a little extreme (as revolutionaries tend to be) and they held onto their position of power for only a decade or so. While
some of their legacy (including their political art, rather mediocre as is much political art) has been forgotten, their clever and sensible metric
system endures. Most scientists, and most countries, use the metric system today.
Scientists continue to update and refine the metric system. This expanded and updated system of measurement used today is called the
Système International d’Unités, or SI. We typically use SI units in this textbook; several times, though, we refer to different units that may be
better known to you or are commonly used in the sciences. We will discuss some of the SI units further in this chapter.
Over the years, scientists have refined measurement systems, making the definition of units ever more precise. For example, instead of being
Metric system and the Système
International
System defines fundamental units
Larger/smaller units based on powers of
10