UNITS OF MASS
1.9 UNITS OF MASS
Mass expresses the degree to which an object resists a change in its state of rest or motion and is proportional to the amount of matter in the object. Weight is the gravitational force acting upon an object and is proportional to mass. An object weighs much less in the gravitational force on the Moon’s surface than on Earth, but
the object’s mass is the same in both places ( Figure 1.7 ). Although mass and weight are not usually distinguished from each other in everyday activities, it is important for
the science student to be aware of the differences between them.
Table 1.4 Units of the International System of Units, SI
Physical quantity Unit
Unit
Measured name
symbol
Definition
Base units Length
metre
Distance traveled by light in a vacuum in 1 second
Mass kilogram
kg
Mass of a platinum-iridium block located at the International Bureau of Weights and Measures at Sevres, France
Time second
9 192 631 770 periods of a specified line in the microwave spectrum of the cesium-133 isotope
Temperature kelvin
1/273.16 the temperature interval between absolute zero and the triple point of water at 273.16 K (0.01˚C)
Amount of mole
Amount of substance containing as many substance
mol
entities (atoms, molecules) as there are atoms in exactly 0.012 kilograms of the carbon-12 isotope
Electric current ampere
Luminous candela
cd —
intensity Examples of derived units
Force newton
Force required to impart an acceleration of 1 m/s 2 to a mass of 1 kg
Energy (heat) joule
Work performed by 1 newton acting over a distance of 1 meter
Pressure pascal
Pa
Force of 1 newton acting on an area of
1 square meter
The gram (g) with a mass equal to 1/1000 that of the SI kilogram (see Table 1.4 ) is the fundamental unit of mass in the metric system. Although the gram is a conven- ient unit for many laboratory-scale operations, other units that are multiples of the gram are often more useful for expressing mass. The names of these are obtained by affixing the appropriate prefixes from Table 1.5 to “gram.” Global burdens of atmos-
pheric pollutants may be given in units of teragrams, each equal to 1 × 10 12 grams. Significant quantities of toxic water pollutants may be measured in micrograms (1 ×
10- 6 grams). Large-scale industrial chemicals are marketed in units of megagrams (Mg). This quantity is also known as a metric ton, or tonne, and is somewhat larger
(2205 lb) than the 2000-lb short ton still used in commerce in the United States. Table
1.6 summarizes some of the more commonly used metric units of mass and their rela- tionship to some English units.
Table 1.5 Prefixes Commonly Used to Designate Multiples of Units
Prefix Basic unit is multliplied by Abbreviation
Mega
M Kilo
k Hecto
h Deka
da Deci
d Centi
0.1 (10- 1 )
c Milli
0.01 (10- 2 )
m Micro
n Pico
0.000 000 001 (10- 9 )
0.000 000 000 001 (10- 12 )
A cannonball resting on The same cannonball propelled an astronaut’s foot in an
across the spaceship cabin and orbiting spacecraft would
striking the astronaut would be cause no discomfort
painful because of the momen- because it is weightless
tum of its mass in motion in outer space.
Figure 1.7 An object maintains its mass even in the weightless surroundings of outer space.