SI System: Base and Derived Units

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Chapter 2.1

• SI System Base and Derived Units

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SI System

• Le Systeme International dUnites
• International System of Units
• Often called the metric system

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Why do we need this?

• When no one left their own community, it didnt
  really matter.
• 1.5 killmers of cookies would be a perfectly
  acceptable unit of measure, if everyone had a
  killmer to refer to.
• As we become more global, common means of sharing
  ideas becomes more significant.

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The SI System Base Units
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Sometimes the units dont match the size we need

• It doesnt make sense to measure the thickness of
  a piece of paper with a meter-stick
• So we subdivide the units, rather than make up
  new ones

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Some common prefixes
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Lets try some

• How many grams in a kilogram?
• How many centimeters in a meter?
• How many deciliters in a liter?
• How many killmers in a hectokillmer?

• 1000
• 100
• 10
• 100

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We find these all over

• What does the k in Y2k mean?
• How many years in a century?
• How many years in a millenium?
• How many tons in a 10 megaton bomb?

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Approximating

• 1 meter is about the height of a doorknob.
• 1 cm is about the width of a button.
• 1 mm is about the thickness of a dime.
• 1 kg is about the mass of a small text book.
• 1 g is about the mass of a dollar bill.
• 1 mg is about the mass of 10 grains of salt.

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Base Units vs. Derived Units

• Base units are the simplest units.
• We can combine base units to make derived units.
• A derived unit is used to measure those things
  that are not part of the base options

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Derived Unit

• If I want to measure the volume for a cube, I
  need to use a derived unit.
• V length x length x length
• V m x m x m
• V m3
• A cubic meter is a derived unit built from base
  units.

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Density

• One of the first needs we will have for derived
  units involves density.
• Density is the amount of mass in a given volume.
• D mass / volume

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Unit for density

• Since density doesnt have a base unit, we have
  to build one.
• D mass / volume
• D kg / m3
• The nice part is, we can use any mass or volume
  units to do this

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Density Sample

• What is the density of a block of metal that has
  a mass of 2.50 grams and occupies a volume of .20
  ml?
• D mass/volume
• D (2.50 g)/(.20 ml)
• D 12.50 g/ml

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Rearranging the formula

• We can use algebra tricks to rearrange our
  density formula if we need to
• D mass/volume
• VolumeD mass
• Volume mass/D

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Assignment

• Read Lab 2 for next time