**Physical Quantity** is a quantity that **can be measure****d** and it consists of a **numerical magnitude** (size) and a **unit**. (e.g. 5m where 5 is the numerical magnitude and m is the unit)

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

**Base Quantity** |
**Name of SI unit** |
**Symbol for SI unit** |

Length |
metre |
m |

Mass |
kilogram |
kg |

Time |
second |
s |

Electric Current |
ampere |
A |

Thermodynamic temperature |
kelvin |
K |

Luminous intensity |
candela |
cd |

Amount of substance |
mole |
mol |

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**

**Physical Quantity** |
**How it is derived from base quantities** |
**Symbol for SI unit** |

Area |
length x width |
m^{2} |

Volume |
length x width x height |
m^{3} |

Speed |
length/time |
m/s |

Acceleration |
velocity/time |
m/s^{2} |

Density |
mass/volume |
kg/m^{3} |

You can think of base quantities as the smallest building blocks (like Lego bricks). When you combined any base quantities to ‘build’ into something else, they becomes a derived quantity.The derived quantity is ‘derived’ or ‘gotten from’ 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!

**Factor** |
**Prefix** |
**Symbol** |

10^{9} |
giga- |
G |

10^{6} |
mega- |
M |

10^{3} |
kilo- |
k |

10^{-1} |
deci- |
d |

10^{-2} |
centi- |
c |

10^{-3} |
milli- |
m |

10^{-6} |
micro- |
µ |

10^{-9} |
nano- |
n |