3
The Metric System
Functions
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3.3
Mass and Weight
Copyright © Cengage Learning. All rights reserved.
Mass and Weight
The mass of an object is the quantity of material making up
the object.
One unit of mass in the SI system is the gram (g).
The gram is defined as the mass contained in 1 cubic
centimetre (cm3) of water, at its maximum density.
A common paper clip has a mass of about 1 g. Three
aspirin have a mass of about 1 g. (See Figure 3.8.)
(a) A common paperclip has a mass of about 1 g.
(b) Three aspirin have a mass of about 1 g.
Figure 3.8
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Mass and Weight
Because the gram is so small, the kilogram (kg) is the
basic unit of mass in the SI system.
One kilogram is defined as the mass contained in 1 cubic
decimetre (dm3) of water at its maximum density.
For very large quantities, such as a trainload of coal or
grain or a shipload of ore, the metric ton (1000 kg) is used.
The milligram (mg) is used to measure very, very small
masses such as medicine dosages. One grain of salt has a
mass of about 1 mg.
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Mass and Weight
The weight of an object is a measure of the earth’s
gravitational force—or pull—acting on the object.
The SI unit of weight is the newton (N).
The terms mass and weight are commonly used
interchangeably by the general public.
We have presented them here as technical terms, as they
are used in the technical, engineering, and scientific
professions.
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Mass and Weight
To further illustrate the difference, the mass of an astronaut
remains relatively constant while his or her weight varies
(the weight decreases as the distance from the earth
increases).
If the spaceship is in orbit or farther out in space, we say
the crew is “weightless,” because they seem to float freely
in space.
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Mass and Weight
Their mass has not changed, although their weight is near
zero. (See Figure 3.9.)
“Weightless” astronaut.
Figure 3.9
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Example 1
Change 12 kg to grams.
First, kilo means 103 or 1000; and 1 kg = 1000 g.
Choose the conversion factor with kilograms in the
denominator and grams in the numerator.
conversion factor
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