1.6 - Time
Despite the appeal of measuring a lifetime in daylights, sunsets, midnights, cups of
coffee, inches, miles, laughter, or in strife, physicists still choose “seconds.” How
refreshingly simple!
However, as you might expect, physicists have developed a precise way to define a
second. Atomic clocks, such as the one shown in Concept 1 to the right, rely on the fact
that cesium-133 atoms undergo a transition when exposed to microwave radiation at a
frequency of 9,192,631,770 cycles per second. These clocks are extremely accurate.
Thousands of years would pass before two such clocks would differ even by a second.
If you are an exceedingly precise person, you might want to consider buying a
wristwatch that calibrates itself via radio signals from an atomic clock. For now, though,
you can visit a web site that displays the current time as measured by an atomic clock.
In addition to being used to measure a second, atomic clocks are used to keep
time. The length of a day on Earth, measured by the time to complete one rotation, is
not constant. Why? The frictional force of tides causes the Earth to spin more slowly.
This means that the day is getting longer (does it not just feel that way sometimes?).
Every fifteen months or so since 1978, a leap second has been added to official time-
keeping clocks worldwide to compensate for increased time it takes the Earth to
complete a revolution.
Time
Measured in seconds (s)
1 second = 9,192,631,770 cycles1 hour = 3600 seconds
1 day = 86,400 seconds1.7 - Mass
The standard unit of mass is the kilogram. (The British system equivalent is the slug,
which is perhaps another reason to go metric.)
Physicists define mass as the property of an object that measures its resistance to a
change in motion. A car has more mass than a bicycle. The three people shown
straining at the car above will cause it to accelerate slowly; if they were pushing a
bicycle instead, they could increase its speed much more quickly. Once they do set the
car in motion, if they are not careful, its mass might prevent them from stopping it.
The official kilogram, the International Prototype Kilogram, is a cylinder of platinum-
iridium alloy that resides at France’s International Bureau of Weights and Measures.
Copies of this kilogram reside in other secure facilities in different countries and are
occasionally brought back for comparison to the original.
A liter of water has a mass of about one kilogram. A typical can of soda contains about
354 milliliters and has a mass of 0.354 kilograms.
It is tempting to write that one kilogram equals about 2.2 pounds, but this is wrong. The
pound is a unit of weight; kilograms and slugs are units for mass. Weight measures the force of gravity that a planet exerts on an object, while
mass reflects that object’s resistance to change in motion. A classic example illustrates the difference: Your mass is the same on the Earth and
the Moon, but you weigh less on the Moon because it exerts less gravitational force on you. On Earth, the force of gravity on one kilogram is
2.2 pounds but the force of gravity on a kilogram is only 0.36 pounds on the Moon.
Kilogram is abbreviated as kg. We typically use kilograms in this book, not grams (which are abbreviated as g).
The three units you need in order to understand motion, force and energy, the topics that start a physics textbook, are meters, kilograms and
seconds. Other units used in studying these topics are derived from these fundamental units.
Mass
Measured in kilograms (kg)
Resistance to change in motion
Not weight!One kilogram = one liter of water
One gram = about 25 raindrops