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1.2 Physical quantities and units  (Page 4/18)

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Metric prefixes for powers of 10 and their symbols
Prefix Symbol Value See Appendix A for a discussion of powers of 10. Example (some are approximate)
exa E 10 18 size 12{"10" rSup { size 8{"18"} } } {} exameter Em 10 18  m size 12{"10" rSup { size 8{"18"} } " m"} {} distance light travels in a century
peta P 10 15 size 12{"10" rSup { size 8{"15"} } } {} petasecond Ps 10 15  s size 12{"10" rSup { size 8{"15"} } " s"} {} 30 million years
tera T 10 12 size 12{"10" rSup { size 8{"12"} } } {} terawatt TW 10 12  W size 12{"10" rSup { size 8{"12"} } `W} {} powerful laser output
giga G 10 9 size 12{"10" rSup { size 8{9} } } {} gigahertz GHz 10 9  Hz size 12{"10" rSup { size 8{9} } `"Hz"} {} a microwave frequency
mega M 10 6 size 12{"10" rSup { size 8{6} } } {} megacurie MCi 10 6  Ci size 12{"10" rSup { size 8{6} } `"Ci"} {} high radioactivity
kilo k 10 3 size 12{"10" rSup { size 8{3} } } {} kilometer km 10 3  m size 12{"10" rSup { size 8{3} } " m"} {} about 6/10 mile
hecto h 10 2 size 12{"10" rSup { size 8{2} } } {} hectoliter hL 10 2  L size 12{"10" rSup { size 8{2} } " L"} {} 26 gallons
deka da 10 1 size 12{"10" rSup { size 8{1} } } {} dekagram dag 10 1  g size 12{"10" rSup { size 8{1} } `g} {} teaspoon of butter
— — 10 0 size 12{"10" rSup { size 8{0} } } {} (=1)
deci d 10 − 1 size 12{"10" rSup { size 8{ - 1} } } {} deciliter dL 10 − 1  L size 12{"10" rSup { size 8{ - 1} } `L} {} less than half a soda
centi c 10 − 2 size 12{"10" rSup { size 8{ - 2} } } {} centimeter cm 10 − 2  m size 12{"10" rSup { size 8{ - 2} } `m} {} fingertip thickness
milli m 10 − 3 size 12{"10" rSup { size 8{ - 3} } } {} millimeter mm 10 − 3  m size 12{"10" rSup { size 8{ - 3} } `m} {} flea at its shoulders
micro µ 10 − 6 size 12{"10" rSup { size 8{ - 6} } } {} micrometer µm 10 − 6  m size 12{"10" rSup { size 8{ - 6} } `m} {} detail in microscope
nano n 10 − 9 size 12{"10" rSup { size 8{ - 9} } } {} nanogram ng 10 − 9  g size 12{"10" rSup { size 8{ - 9} } `g} {} small speck of dust
pico p 10 − 12 size 12{"10" rSup { size 8{ - "12"} } } {} picofarad pF 10 − 12  F size 12{"10" rSup { size 8{ - "12"} } F} {} small capacitor in radio
femto f 10 − 15 size 12{"10" rSup { size 8{ - "15"} } } {} femtometer fm 10 − 15  m size 12{"10" rSup { size 8{ - "15"} } `m} {} size of a proton
atto a 10 − 18 size 12{"10" rSup { size 8{ - "18"} } } {} attosecond as 10 − 18  s size 12{"10" rSup { size 8{ - "18"} } `s} {} time light crosses an atom

Known ranges of length, mass, and time

The vastness of the universe and the breadth over which physics applies are illustrated by the wide range of examples of known lengths, masses, and times in [link] . Examination of this table will give you some feeling for the range of possible topics and numerical values. (See [link] and [link] .)

A magnified image of tiny phytoplankton swimming among the crystal of ice.[
Tiny phytoplankton swims among crystals of ice in the Antarctic Sea. They range from a few micrometers to as much as 2 millimeters in length. (credit: Prof. Gordon T. Taylor, Stony Brook University; NOAA Corps Collections)
A view of Abell Galaxy with some bright stars and some hot gases.
Galaxies collide 2.4 billion light years away from Earth. The tremendous range of observable phenomena in nature challenges the imagination. (credit: NASA/CXC/UVic./A. Mahdavi et al. Optical/lensing: CFHT/UVic./H. Hoekstra et al.)

Unit conversion and dimensional analysis

It is often necessary to convert from one type of unit to another. For example, if you are reading a European cookbook, some quantities may be expressed in units of liters and you need to convert them to cups. Or, perhaps you are reading walking directions from one location to another and you are interested in how many miles you will be walking. In this case, you will need to convert units of feet to miles.

Let us consider a simple example of how to convert units. Let us say that we want to convert 80 meters (m) to kilometers (km).

The first thing to do is to list the units that you have and the units that you want to convert to. In this case, we have units in meters and we want to convert to kilometers .

Next, we need to determine a conversion factor    relating meters to kilometers. A conversion factor is a ratio expressing how many of one unit are equal to another unit. For example, there are 12 inches in 1 foot, 100 centimeters in 1 meter, 60 seconds in 1 minute, and so on. In this case, we know that there are 1,000 meters in 1 kilometer.

Now we can set up our unit conversion. We will write the units that we have and then multiply them by the conversion factor so that the units cancel out, as shown:

80 m × 1 km 1000 m = 0 .080 km. size 12{"80"" m" times { {"1 km"} over {"1000 m"} } =0 "." "080"`"km"} {}

Note that the unwanted m unit cancels, leaving only the desired km unit. You can use this method to convert between any types of unit.

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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