SI Base Units and Derived Units

The International System of Units (SI) provides a universal language for measurement in physics. Every physical quantity is expressed using combinations of seven base units.

What are the SI base units?

Five base quantities appear regularly in IGCSE Physics 0625:

Base quantity	Base unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Temperature	kelvin	K
Electric current	ampere	A

The remaining two (amount of substance: mole, luminous intensity: candela) are outside the 0625 syllabus.

What are derived units?

Derived units are combinations of base units. Speed is measured in metres per second (m/s), which combines length and time. Force is measured in newtons, where $1\ \text{N} = 1\ \text{kg}\cdot\text{m/s}^2$.

Common derived units in IGCSE Physics:

Quantity	Derived unit	In base units
Speed	m/s	m/s
Acceleration	m/s²	m/s²
Force	newton (N)	kg m/s²
Energy	joule (J)	kg m²/s²
Power	watt (W)	kg m²/s³
Pressure	pascal (Pa)	kg/(m s²)

Prefixes for large and small values

Prefix	Symbol	Multiplier
kilo	k	×1000
centi	c	×0.01
milli	m	×0.001
micro	μ	×0.000001

To convert: $3.5\ \text{km} = 3.5 \times 1000 = 3500\ \text{m}$.

Common errors and how to correct them

• Confusing kg and g in calculations. The SI base unit for mass is the kilogram, not the gram. Always convert grams to kilograms before substituting into equations.
• Using cm instead of m for length. Many exam questions give lengths in centimetres. Divide by 100 to convert to metres before calculating area or volume in SI units.
• Forgetting that the newton is a derived unit. Students sometimes treat it as a base unit. Remember: $1\ \text{N} = 1\ \text{kg m/s}^2$.

How to apply this in an exam

When a question gives values in non-SI units, convert first. Write the conversion step explicitly. Examiners can award method marks only if the working is visible. Check the unit of the final answer against the unit implied by the equation.