**Physical Quantity** is a quantity that

**and it consists of a**

__can be measured__**(size) and a**

__numerical magnitude__**. (e.g. 5m where 5 is the numerical magnitude and m is the unit)**

__unit__**1) The seven base quantities and their SI units**

Out of the seven, for O level Physics, you are required to learn the first 5 *(length, mass, time, current and temperature)*.

**2) Some common derived quantities and units**

You can think of base quantities as the smallest building blocks (like Lego bricks). When you combined any base quantities to â€˜buildâ€™ into something else, they becomes a derived quantity.The derived quantity is â€˜derivedâ€™ or â€˜gotten fromâ€™ when 2 or more quantities are combined together.

__3) Prefixes for SI units__

Prefixes are used to represent very large or small quantities.For example, instead of writing 1000000 m, it will be more convenient to write 1Mm, which means the same thing!

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