Quantities And Units

There are two groups of quantities and units in physics, namely

• the fundamental quantities and units
• the derived quantities and units

1. The fundamental quantities and units are the basic quantities and units in physics that are independent of other quantities and units. This simply means that other quantities and units that ain't among the fundamental ones depends on the fundamentals for their derivation.

"Hold on, i sense confusion in that explanation! But don't worry its pretty cheap you'll understand it before leaving this page..."

Examples of fundamental quantities in physics includes

1. distance/Length (breadth, width, height)
2. Mass
3. Time
4. Electric current
5. Temperature
6. Amount of substance

However the most important fundamental quantities in physics are:  Mass, Length and Time

  "For easy memorizing, abbreviate them as: M L T 

(you can pronounce it like 'MELT')"

Table 1.1 fundamental quantities and units

Quantity                                 Unit                  Unit's sign
distance/Length                      metre                     M       
Time                                     second                    S       
Mass                                     kilogram                 Kg      
Electric current                       ampere                   A       
Temperature                           kelvin                      K      
Amount of substance              mole                       mol    

note: The units in the table above are all in the SI unit form.

"System International (SI) Units: Are the generally accepted standard units used in scientific works all over the world, especially in the laboratory."

After the measurement of a particular thing is concluded, a simple word is attached at the end of the measurement value to give a complete meaning.

For instance; when you go to a shop to buy a physics text book, the cashier doesn't just say "am sorry boy but the price is 5"  he (the cashier) needs to elaborate more in order to make a complete sense i.e he needs to tell you if he meant 5 dollars, 5 cent or 5 pounds thus the dollar, cent and pound here, are the different units of the quantity "5".


Observe the currency below,  identify its quantity and its unit

we can identify that the quantity is "5" and the unit is "Dollars", not pounds not cent not Naira and not Euro.

But this case is just a commercial example. In science, it still takes a similar format.
For instance; after measuring the length of your dinning table with a tape or a meter ruler, you need to convey the message in a manner that it is completely understood i.e

#. you don't just say...

"the length of the dinning table is 10" {meaningless

#. instead you say...

"the length of the dinning table is 10 meters". {meaning completed

The "meter" added here is the unit of the quantity "10" and this completes the meaning of your report.
Length has many units e.g meters, centimeters, miles, kilometers etc. so when giving a report based on length you need to tell the particular unit you are referring to.

the length of your  dinning table could be 10 meters or 10 centimeters or 10 millimeters... etc

Note: A particular quantity can only have one unit (there can never be anything like "the length of the dining table is 10 metre centimeter.")

2  Derived quantities and units are those quantities and units obtained by some simple combination of two or more fundamental quantities and units.

Examples of derived quantities in physics includes

1. Volume
2. Area
3. Velocity
4. Acceleration
5. Force
6. Work, etc.

Sample 1
* Volume is a derived quantity obtained by the combination of three fundamental quantities: Length, Breadth and Height.

length(m)  x  breadth(m)  x  height(m)

Sample 2

* Area is a derived quantity obtained by the combination of two fundamental quantities: Length and Width

length(m)  x  width(m)

Sample 3

* Velocity is a derived quantity gotten from a simple combination of two fundamental quantities: distance and time.

   the derived unit for velocity is   

Sample 4

* Acceleration is a derived quantity obtained by the combination of two fundamental quantities: distance and time

    the derived unit for acceleration is    

Sample 5

* force is a derived quantity obtained by the combination of three fundamental quantities: mass, distance and time

  the derived unit for force is    also known as Newton

Sample 6

* work is a derived quantity obtained by the combination of three fundamental quantities: mass, distance and time

  the derived unit for work is      also known as Joule

Observe that: Fundamental quantities have fundamental units while derived quantities have derived units.