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Base Units & Physical Quantities

Physical quantities

It is a quantity that can be expressed in terms of magnitude and units and is measured using scientific instruments. Eg - Mass

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 = FgR2/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 10x

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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