The SI System

or

How to Know “How Much?”

The Metric System

• Officially called the SI System• Based on increments of 10 (one decimal place or

zero)• A good measurement has a number and a unit• Prefixes on the unit name tell you how many zeroes to

put before or after a number• Used in every country around the world except the

USA and Báhrain• The official measurement system of the USA

since the late 1800s

Why you don’t already know the Metric System

Your parents didn’t know it

No incentive for companies to switch

You are still being taught the Imperial

System through everyday experiences

- Recipes

- Consumer goods

There is a slow, steady inclusion process

Base Units

• Base units are defined by only one measurement

Length

• Distance from one point to another• Meters (m) are the base units (a little longer

than a yard)

• For smaller items, centimeters (about one knuckle), and millimeters (the thickness of a dime) are used

Mass

• Amount of substance (number of particles) in an object

• Gram (g) is the base unit• It is too small for some common measurements

(about 1 paper clip of matter)• Usually use the kilogram (kg) for basis of

comparisons• A kilogram is a little more than 2 pounds

Time

• How long it takes for something to happen• Also called “Time Elapsed” or “Elapsed Time”

• Seconds (s) are the base unit• Can lead to large numbers (1hr = 3600s)

What would you use to measure the length of your arm?

1. Liters

2. Meters

3. Grams

4. Grams/Milliliter

What would you use to measure how much sugar is in one teaspoon?

1. Liters

2. Meters

3. Grams

4. Kilograms

What would you use to measure how much you weigh?

1. Liters

2. Meters

3. Grams

4. Kilograms

What would you use to measure the width of the room?

1. Liters

2. Meters

3. Grams

4. Grams/Milliliter

Derived Units of the SI System

• Derived units are defined by two or more measurements

• Volume• Amount of space taken up by a substance

• Liters (L) are the base units (a little more than a quart)

• Liters are too large for some measurements, so milliliters (mL) are often used

What would you use to measure how much water is in a pool?

1. Liters

2. Meters

3. Grams

4. Grams/Milliliter

More Derived Units

• Density• Amount of matter in an object divided by the space

taken up by the object

• “How much stuff is in how much space”

• Usually expressed in grams per milliliter (g/mL) for smaller amounts

• Expressed in kilograms per liter (kg/L) for larger amounts

• THE NUMBER DOES NOT CHANGE

SI Prefixes

• Used to show a number of base units

• Think of them as coefficients (multipliers)

• Easy to convert by sliding the decimal

SI Prefixes

• From the largest to the smallest• kilo- hecto- deka- BASE deci- centi- milli-• 1000 100 10 1 1/10 1/100 1/1000• “King Henry died by drinking chocolate milk.”• Put your finger on the unit you have, move it to

the unit you want• Move your decimal the same number of places in

the same direction that your finger moved

Testing your knowledge…

• Answer the five questions…

• Use the “Metric Ruler” to slide the decimal to get the desired units…

• Ready?

What is 25mm in m?

1. 2.5m

2. 0.25m

3. 0.025m

4. 0.0025m

How many grams are 1.12kg?

1. 112g

2. 0.00112g

3. 0.112g

4. 1120g

How many Liters are 591mL?

1. 0.591L

2. 59.1L

3. 0.059L

4. 5.91L

What is 2.5cm in mm?

1. 250mm

2. .25mm

3. 25mm

4. 0.025mm

0.355L is how many mL?

1. 3.55mL

2. 35.5mL

3. 355mL

4. 3550mL

Volume and Density in the SI System

Why things float or sink, metrically

Metric Density and Water

• The SI system is better than the English/Imperial system at converting length measurements to volume measurements.

• The density of water is defined as 1g of water occupying 1cm3 of water at 4C. The density of water is 1 g/cm3.

Common SI Volume Conversions

• 1cm3 = 1mL

• 1000cm3 = 1000mL

• 1L = 1000mL

• 1000mL = 1L = 1000cm3

Density Formula

• Knowing the volume, mass, and/or density of a substance will allow you to infer as to the missing quantity.

• Density = mass d = m

volume v

• m = d • v v = m

d

Some Common Densities in g/cm3

Air 0.00129 Titanium 4.5

Wood – Cherry 0.433 Iron 7.86

Gasoline 0.721 Steel 8.03

Water – ICE 0.897 Brass 8.553

Mineral Oil 0.914 Copper 8.9

Paper 0.929 Silver 10.5

Water - 4C 1.000 Lead 11.37

Glass 2.6 Uranium 18.74

Aluminium 2.643 Gold 19.32

Sapphire 3.99 Platinum 21.3

Iridium, a rare metal found in asteroids, comets, and meteorites has the highest natural density at 22.16 g/cm3.

Will it float?

• When any two substances are mixed, the less dense substance will float on the more dense substance• This causes rainbows on water and convection.

Significant Digits

• 1. Zeros appearing between nonzero digits are significant.• 3005

• 2. Zeros appearing in front of all nonzero digits are not significant.• 0.00629

• 3. Zeros at the end of a number & to the right of a decimal point are significant.• 2750 & 2750.01

Significant Digits

• 4. Zeros at the end of a # but to the left of a decimal point may or may not be significant.• 30 or 30.