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.