W h y do w e measur
Metric
How mu&? When you shop, you want to
get the most for your m o m . When you.
buy ehopped meat, youwant to know how
miieh meat is fa the .package;. The we1(in
the package tells yon how much it weighs.
Weight isone waydfdescrib'ingthe ammttt
of sometlnng, Weight is one of the ptogerties of matter that can b'e measured,.
Length, area-, and yalume we also pfiaperties, we em measure. T o fiad' out tilings
&Opt mafter, we:'@st des;enl?e its [email protected] properties can be ffle'asured.
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Units of mass. We buy chopped meat by
the pound. The pound is a &t (YOtl-nit)
of weight. Scientists do not use weight to
mean the amount of something. They use
the word mass. They measure mass in
grams. The gram is a unit of mass in the
metric system (MEH-tric SIS-tern). A
nickel has a mass of about 5 grams. Weight
and mass are related, but they are not the
same. Yon will learn more about weight
and mass in another [esson.
b What unit Is used to meMiurf.niaas fn
W'~.:~È<BB~~>
Too muohIstoo many.The gram is ametimes too. small for certain measurements.
The nass,io grainy comes. out in large numbers. A person may have %mass Of'75.0.00
.s of'a ear may be a s much
ms
me
-~m~
as 1,500,000:@&1is. It would help ti have8
'larger unit bf mass. Then the numbers
would be smaller.
~.
Cutting it to size. For large objects, the
gram may be too small. For small objects,
the gram may be too big. The mass of an
insect may be only 11100 of a gram. The
tiic system can solve this prrL'--.
PREFIX
MEANING
oms million (1,000,0001
one thousand (1,0001
me hundred (TOO1
ten 001
m e hundredth
line thouwntl*
1
(14100)
lT/l,OOCIl
Look at the table. A prefix (PREE-fix) can
be used to change the size of a unit. It can
make the unit larger o r smaller. "Kilo-"
(KILL-uh) is a prefix that means 1,000. A
-(KILL-uh-gmm)
is 1,000 grams.
The mass of large objects is measured
in kilograms. A person with a mass of
75,000 grams has a mass of 75 kilograms.
"Centi-" (SEN-tih) is a prefix meaning
11100. An insect with a mass of 11100 of a
gram has a mass of 1 centigram.
What 1s a kilogram?
Just a minute! You have a date at 8:00.
How long must you wait? Scientists say
that the earth is 5 billion years old. How
old are you? Time is not matter. But time
matters! It is important to measure time. In
daily life, we use many different units to
measure time. The second, the minute, the
hour, the day, the year,, and the century
(100 years) are units of time. The second is
the mainunit of time in the metric system.
What Is the unit tar measuring time In
the metric system?
-(AT YOU LEARNED
1. The amount of matter is called its mass,
2. In the metnc system, mass is measured in
grams.
3. Prefixes are used to make units larger or
smaller.
4. Time is measured in seconds.
SCIENCE WORDS
milt (YOU-nit)
an amount that is used to measure things
gram
a unit of mass m the metnc system
metric system (MEH-trick SIS-tem)
the units of measurement used by scientists
kilogram (KILL-uh-gram)
1,000 grams
ANSWER THESE
1. Write the following masses in order from
the smallest to the largest.
milligram
kilogram
microgram
hectogram
centigram
gram
2. Use the table of prefixes to find the meaning
of each of these:
microsecond
millisecond
megaton
NOW TRY THESE
1. How many seconds are there in a century?
Give your answer in megaseconds.
2. Scientists sometimes measure the cost of
scientific equipment i n "megabucks." A
large atom-smasher may cost 500
megabucks to build. What do you think that
means7
FINDING OUT MORE
The International Bureau of Weights and Measures is located near Pans. France. The standard unit of mass is kept there. This mass is
exactly 1,000grams or 1 kilogram. Each nation
keeps a copy of this mass in its own national
laboratory. This is done so that scientists all
over the world can be sure they are measuring
mass in the same units. The standard kilogram
is made of a platinum-iridium alloy that will not
rust. It is carefully protected against damage.
What might happen if this standard mass became scratched?