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The Metric System
Functions
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3.3
Mass and Weight
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3. (a) A common paperclip has a mass of about 1 g.
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

4. 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.

5. 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.

6. 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.

7. Mass and Weight
Their mass has not changed, although their weight is near zero. (See Figure 3.9.)
“Weightless” astronaut.
Figure 3.9

8. 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