Physical quantity refers to any property of matter that can be measured. (or simply a quantity).

Scalar quantities - a quantity that depends on only magnitude, e.g speed
Vector quantities - a quantity that depends on magnitude and direction, e.g velocity.

What is measuring?
When you measure an amount of a quantity, you can compare that amount with a standard amount of the same quantity.
This standard amount is called a unit.

SI Units
In the 1960s, scientists agreed to use a particular system of units.
They called this system the International System of Units.
Any unit of this system is called an SI unit.

Derived Units

The unit of every other quantity is called a derived unit because it can be written as a product or quotient of one or more base units.

Examples of Derived Units

Sometimes a unit is given the name of a scientist who made discoveries in the area of science where that unit often occurs.

When this happens the unit is written with a capital letter but the name of the unit is not.

Using SI Units

Value of each quantity is expressed in the correct SI Unit before you start.

When you calculate an answer you write it with the correct unit after it.

when converting to scientific notation: first find the power for the unit (e.g. if going from cm to m it will be *10^-2). if the number is more than one digit (e.g. 40*10^-2 m) then move the decimal place by one and ADD one to the indices (e.g.4×10^-1 m)
if it is SQUARED: find the power for the unit (e.g. going from mm² to m² it will be *10^-3) then multiple the power by 2 (e.g. *10^-3 becomes *10^-6).
if it is CUBED: find the power for the unit (e.g. going from mm³ to m³ it will be *10^-3) then multiple the power by 3 (e.g. *10^-3 becomes *10^-9).