Chapter 1 Legacy

On Units of Measure

The Metric System• In the past, many different units were used to

measure things (ex. Foot, cubit, furlong, fathom)

• However, these units cannot be universally understood and repeatable, so scientists established the International System of Units (SI), better known as the metric system.

• First, it is important to understand the meanings of QUANTITIES and UNITS. It can sometimes be difficult to tell the difference.

Quantities and Units

Quantities• The amount of a specific

dimension• Includes Length, Mass,

Time, Temperature, etc.

Units• The standard sizes that you

measure quantities in• Includes meter, kilogram,

second, foot, pound, mile, etc.

Fundamental vs. Derived Units

• Fundamental units are base units that form the building blocks of all other measured units. They are universally understood and repeatable.

• Derived units are units made up of fundamental units, ex. m/s, kg/m^3, g*cm/s^2

There are 7 fundamental units/quantities in all…

• You should already be familiar with 3…– Meter (Length)– Kilogram (Mass)– Second (Time)

• And then there are these…– Ampere (Electrical Current)– Kelvin (Temperature)– Mole (Amount of Substance)– Candela (Luminous Intensity)

If you needed to, you could try using a pneumonic device to remember.

• My• Killer• Snake• Ate• Kittens• Moles and• Cows

Defining Fundamental Units• A meter, for example, is today defined as the

distance light travels in 1/2999792458 of a second• This may seem ridiculously complex, but it is

necessary to define it this way to make it universally understood and repeatable

• The speed of light is one thing commonly incorporated into these definitions because it can be universally understood and repeatable anywhere in the universe.

Systems of Measurement• There are 3 popularly used systems of measurement, the

third being British. They’re named by the primary fundamental units of the system and abbrieviated.

• mks- meter, kilogram, second• cgs- centimeter, gram, second• fps- foot, pound, second• *It is important to realize that technically, a pound is not a

unit of mass, but of force. Your weight in pounds is subject to change under different gravity (like on the moon), but your mass in kilograms isn’t, no matter where you are.