- recall the following SI base quantities and their units: mass (kg), length (m), time (s), current (A), temperature (K), amount of substance (mol), luminous intensity (cd)
express derived units as products or quotients of the SI base units and use the named units listed in this syllabus as appropriate

use SI base units to check the homogeneity of physical equations

use the following prefixes and their symbols to indicate decimal submultiples or multiples of both base and derived units: pico (p), nano (n), micro (μ), milli (m), centi (c), deci (d), kilo (k), mega (M), giga (G), tera (T)

understand and use the conventions for labelling graph axes and table columns as set out in the ASE publication Signs, Symbols and Systematics (The ASE Companion to 16–19 Science, 2000)

**Système International (SI)**

Système International (SI) is the modern metric system.

- SI has
**7 base quantities**: mass (kg), length (meter, m), time (second, s), current (ampere, A), temperature (Kelvin, K), amount of substance (mole, mol), luminous intensity (candela, cd).

SI units are the building blocks for other physical quantities -

- all other physical quantities called
**derived units**can be expressed as some combination of the 7 base units.

E.g. Acceleration: *meter / second ^{ }*

^{2}

*, Force(Newton, N):*

^{ }*kg meter / second*

^{2 }, Energy joule(J):

*kg meter*

^{2}

^{ }/ second^{- }^{2}

If you start with some apples, you will never end up with bananas. Adding oranges and apples makes no sense - you must add same things; therefore:

- we require each term in a equation must have
*same*unit.

E.g. for uniform accelerating motion: v = u + at , where all terms have the unit of speed. (check it!)

When each term has the same base units on the both sides of a equation, we say it is a homogeneous or balanced equation. In other words, when *distance *is at the left hand side some, but you end up with *time* on the right hand side, you must have made a mistake in the calculation.

E.g. E = mc^{2} : where LHS : joule = kg m^{2} s^{-2 }(Joule expressed in base units) ; RHS : kg x (ms^{-1})^{2 } = kg m^{2}s^{-2} ; it is balanced. However, a balanced equation does not guarantee correctness, but an unbalanced one (that is, for an example, when you find some terms in *meters*, other terms in *second *)* *is doomed to be wrong. There is just no way to get oranges from piling apples.

**Graph Convention**

In IGCSE CIE Physics, there is a convention for writing column of data in tables and graph axes:

*Symbols*(sloping, in italic)**/**Unit (in roman)

E.g. suppose we have record the speed of a car:

*v* / ms-1 : 30 , 60 , 90 , 120

*t* / s : 0 , 1 , 2 , 3

**Multiplies and sub-multiplies**

The quantities can have multiplies and sub-multiplies to cater for larger or smaller value.

E.g. *km* is simply 10^{3} meters.

E.g. the volume of a tiny cube: 10 mm^{3} = 10 x (10^{-3})^{3 }x m^{3 }= 10^{-8} m^{3 }

That's all, see you next time! 🤘

Reference:

Richard Woodside, Chris Mee - Cambridge International AS_A Level Physics Revision Education, 2nd Edition

David Sang, Graham Jones, Gurinder Chadha, Richard Woodside - Cambridge International AS and A Level Physics Coursebook, 2nd Edition