2019 A/L Physics Past Paper · Question 01 (English Medium)

SI base units

Which of the following is not a fundamental unit?

Answer and explanation

Answer: (2) J

What the question asks

Four of the five are SI base units.

One is not a base unit.

Find it.

According to international system of units seven basic quantities and seven corresponding units have been approved.

The seven base units:

length: metre (m)

mass: kilogram (kg)

time: second (s)

temperature: kelvin (K)

current: ampere (A)

amount of substance: mole (mol)

luminous intensity: candela (cd)

Other quantities such as energy and acceleration which can be derived using basic units are called derived units.

Check each option against the list.

m, cd, K and mol are all on the list.

J, the joule, is not.

The joule is the unit of work and energy.

$W = F imes d$

$\text{J} = \text{N m} = \text{kg m}^2\text{ s}^{-2}$

So the joule is built from the base units kg, m and s.

It is a derived unit.

The metre is the base unit of length.

It is the first on the list.

The candela is the base unit of luminous intensity.

The kelvin is the base unit of thermodynamic temperature.

The mole is the base unit of amount of substance.

$\text{J} = \text{kg m}^2\text{ s}^{-2}$

The joule (J) is not a base unit.

It is built from kg, m and s.

The answer is (2).

Learn the seven base units: m, kg, s, K, A, mol and cd.

Units with their own names, such as N, J, W, Pa, C and V, are all derived units.

Exam tip