Transcript Units

Units
Unit Systems
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Systems set up fundamental units.
• British system - foot, pound, second
• Metric system - meter, kilogram, second
Time
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The unit of time originally was based on the day and
the year.
The second was 1/60 * 1/60 *1/24 of a day.
In the 20th century the second was measured based
on the timing of atoms.
We now know that the day is getting longer and “leap
seconds” are added every few years.
The SI unit of time is the second (s)
Length
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The oldest standards of length were based on the
human body.
The metric system defined the meter in terms of the
Earth: 1/10,000,000 from the pole to the equator.
The meter is now defined in terms of the second and
speed of light.
The SI unit of length is the meter (m)
Mass
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Standard weights have been maintained for
centuries.
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Weight and mass were thought to be the same.
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Now a standard 1 kilogram mass is kept in Paris.
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The SI unit of mass is the kilogram (kg)
SI Prefixes
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Prefixes on units are used to
represent powers of ten.
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Prefixes denote powers of
ten from -18 to +18 in steps
of three.
Example: a kilometer is 103
meters or 1000 meters.
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Most Common
micro () 10-6
milli (m) 10-3
kilo (k) 103
mega (M) 106
Common, but the power is
not a factor of three.
centi (c ) 10-2
deci (d ) 10-1
Other Units
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There are other fundamental
units in SI. See NIST.
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ampere (A)
kelvin (K)
mole (mol)
candela (cd)
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Derived units are built from
the fundamental units.
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area (m2)
volume (m3)
velocity (m/s)
acceleration (m/s2)
force (kg m/s2) or (N)
energy (kg m2/s2) or (J)
Density
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Matter has mass and takes
up volume.
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The ratio of the mass to the
volume is the density.
  m /V
Salt (solid): 2.165 x 103 kg/m3
Water (liquid): 1.000 x 103 kg/m3
Nitrogen (gas): 1.251 kg/m3
Matching Units
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Conversion between units must be of the same type.
Length conversion:
• 1 in = 2.54 cm
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Time conversion:
• 1 hr = 3.6 x 103 s
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No conversion between different types of units.
• 1 in is not equivalent to some seconds
Conversion Factors
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A value is converted by applying the ratio of the
conversion factors.
• How many inches in 50. cm?
• 50. cm (1 in / 2.54 cm) = (50. / 2.54) in = 20. in
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Many conversion factors use scientific notation.
• How many seconds in a year?
• 1 yr (365 d/yr) (24 hr/d) (3.6 x 103 s/hr) = 31500 x 103 s =
3.15 x 107 s
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