Measuring with Scientific Units

When something is measured you describe it in terms of numbers and units. The system of measurement used today  throughout most of the world is the SI (Système Internationale d'Unités).  SI symbols are universally understood, regardless of cultural differences in language or alphabet.  This system is commonly called the "metric system" because of its base unit of length the meter.  In all there are seven SI base units:

the meter for distance,

the kilogram for mass,

the second for time,

the ampere for electric current,

the kelvin for temperature,

the mole for amount of substance, and

the candela for intensity of light.

Table 1.  SI base units
SI base unit
Base quantity Name Symbol
length meter m
mass kilogram       kg
time second s

electric current

 ampere A
thermodynamic temperature       kelvin K
amount of substance mole mol
luminous intensity candela cd

SI units are based on multiples of 10.  SI uses prefixes such as kilo- to indicate how many times a unit should be multiplied by ten.  These prefixes are shown in Table 2 below.

 

 Table 2.  SI prefixes
Factor Name  Symbol
1024 yotta Y
1021 zetta Z
1018 exa E
1015 peta P
1012 tera T
109 giga G
106 mega M
103 kilo k
102 hecto h
101 deka da
 
Factor Name  Symbol
10-1 deci d
10-2 centi c
10-3 milli m
10-6 micro µ
10-9 nano n
10-12 pico p
10-15 femto f
10-18 atto a
10-21 zepto z
10-24 yocto y

 

Length

The basic SI unit of length is the meter.

Volume

Volume is the amount of space something occupies.  The unit of volume is the cubic meter, or m3.  In our science activities we will most likely use the cubic centimeter (cm3).  However, the basic SI unit for volume is the liter (L).  Soft drinks often come in 1-liter or 2-liter containers. 

Mass and Weight

Because weight measurements depend on where they are taken, weight can be unreliable for scientific use.  That is why scientists use a measurement called mass.  Mass depends on the number and kinds of atoms that make up an object.  An object's mass always remains the same, no matter where the measurement is taken.  The basic SI unit of mass is the kilogram (kg). 

Density

Take a look at the two boxes below. Each box has the same volume. If each ball has the same mass, which box would weigh more? Why?

The box that has more balls has more mass per unit of volume. This property of matter is called density. Density is defined as the average mass per unit volume; it is a measure of how much matter is squeezed into a given space. The more closely packed the molecules, the higher the density of the materials.  The density of a material helps to distinguish it from other materials. Since mass is usually expressed in grams and volume in cubic centimeters, density is expressed in grams/cubic centimeter.

We can calculate density using the formula:

Density= Mass/Volume

Pure water has a density of 1g/cm3.  An object's density determines whether or not it will float or sink.  The density of everything is compared to the density of pure water.  Materials with densities higher than pure water always sink.  Those with densities less than that of pure water always float.

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