Base Units & Physical Quantities

Base Units

There are 7 SI base Quantities

Base quantities	Base units	Symbol
Mass	Kilograms	kg
Length	Meters	m
Light intensity	Candela	cd
Current	Ampere	A
Amount of substance	Mole	mol
Time	Seconds	s
Temperature	Kelvin	K

These Base quantities are used in physics to derive other units such as kgms^-2

F = ma

kgms^-2 = kg*ms^-2

We will explain what each of the base quantities means later in the chapters

Base units are very important as they help you to find the units of constants

For example below:

So we know that Force is kgms^-2 and so this can be written as:

G = F_gR²/m₁m₂

G = (kgms^-2 * m²)/ kg²

G = kg^-1m³s^-2

This is called the homogenity of an equation.Both sides of the equation must have the same base units.

Remember that when they give Examples of base Quantities. Don't Give examples of Base Units

Multiples and Submultiples prefixes

Usually, it is hard to denote a high magnitude such as 10000000grams. We can write in standard form which shortens it to 1 digit.

10000000grams = 1*10⁷g

There are also standard quantities which could be used instead of using 10^x

To make it easier to understand we will use the unit meters (m) to show the magnitude of the length in terms of meters when different prefixes are used

Prefixes	Symbol - Example	Magnitude
Pico	pm	10-12m
Nano	nm	10-9m
Micro	µm	10-6m
Milli	mm	10-3m
Centi	cm	10-2m
Deci	dm	10-1m
Kilo	km	103m
Mega	Mm	106m
Giga	Gm	109m
Tera	Tm	1012m

So if we need to convert from one unit to another to add them together or compare. Here are some basic ideas.

Let's see how to convert one 1m to pm

So first thing to notice we are converting a big unit to a smaller unit. So the final result must be bigger than the original. Why? In other words, the bigger unit which is m has a bigger capacity that the smaller one and so we need more of the smaller units to equal to the larger one.

1m = 1*10¹²pm

If we were to convert 1 picometer to meters. we will get a smaller magnitude.

1pm = 1*10^-12m

So what this pico does it multiplies the magnitude by 10¹² to get the meters

Just remember when converting from a big unit to a small unit the result must be greater and so vice versa

What if we wanter to convert 1Tg to kg

So using the table we can see that Tera is 10⁹ greater than a kilo. So again use the same principle and the final result must be greater.

Remember 1 tonne is 1000kg

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