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The International System of Units: seven units for the whole world

Your guide: Sir HamzaBelieves every formula has a story, and units never lie.

The problem

A trader in one country sells cloth by his units; the buyer abroad thinks in other units. Scientists had the same trouble sharing results. The answer was one system for everyone.

In 1961 the international committee on weights and measures recommended a system of seven base units, the International System of Units, or SI. It is used all over the world.

Base units cannot be derived from one another. Every other unit, such as square metre or newton, is built from them. These are derived units.

Writing 50000000 m or 0.00004 m is clumsy. SI is a decimal system, so prefixes stand for powers of 10: 5 × 107 m and 4 × 10−5 m.

Table 1.2: the seven base units (p. 8)

Length: metre (m) · Mass: kilogram (kg)

Time: second (s) · Temperature: kelvin (K)

Electric current: ampere (A) · Intensity of light: candela (cd)

Amount of substance: mole (mol)

Table 1.3: some derived units (p. 9)

Area m2 · Volume m3 · Speed m s−1

Force newton (N) · Pressure pascal (Pa)

Electric charge coulomb (C) · Plane angle radian (rad)

Table 1.4: prefixes you use most (p. 10)

pico p 10−12 · nano n 10−9 · micro μ 10−6

milli m 10−3 · centi c 10−2 · deci d 10−1

kilo k 103 · mega M 106 · giga G 109 · tera T 1012

Also: atto 10−18, femto 10−15, peta 1015, exa 1018.

SI has seven base units; every other unit is built from them. A prefix only multiplies a unit by a power of ten.

Step 1 / 7

Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

The problem

A trader in one country sells cloth by his units; the buyer abroad thinks in other units. Scientists had the same trouble sharing results. The answer was one system for everyone.

In 1961 the international committee on weights and measures recommended a system of seven base units, the International System of Units, or SI. It is used all over the world.

Base units cannot be derived from one another. Every other unit, such as square metre or newton, is built from them. These are derived units.

Writing 50000000 m or 0.00004 m is clumsy. SI is a decimal system, so prefixes stand for powers of 10: 5 × 107 m and 4 × 10−5 m.

Table 1.2: the seven base units (p. 8)

Length: metre (m) · Mass: kilogram (kg)

Time: second (s) · Temperature: kelvin (K)

Electric current: ampere (A) · Intensity of light: candela (cd)

Amount of substance: mole (mol)

Table 1.3: some derived units (p. 9)

Area m2 · Volume m3 · Speed m s−1

Force newton (N) · Pressure pascal (Pa)

Electric charge coulomb (C) · Plane angle radian (rad)

Table 1.4: prefixes you use most (p. 10)

pico p 10−12 · nano n 10−9 · micro μ 10−6

milli m 10−3 · centi c 10−2 · deci d 10−1

kilo k 103 · mega M 106 · giga G 109 · tera T 1012

Also: atto 10−18, femto 10−15, peta 1015, exa 1018.

SI has seven base units; every other unit is built from them. A prefix only multiplies a unit by a power of ten.

Key terms

SI
The International System of Units, recommended in 1961.
Base unit
A unit that cannot be derived from the others.
Derived unit
A unit expressed in terms of base units.
Prefix
A word or symbol before a unit that multiplies it by a power of 10.

Short questions with model answers

  1. Q1. Write 2000 μs in milliseconds.

    • 2000 μs = 2000 × 10−6 s
    • = 2 × 10−3 s
    • = 2 ms

    2 ms

  2. Q2. Write 50000 cm in metres.

    • 1 m = 100 cm
    • 50000 cm ÷ 100 = 500 m

    500 m

  3. Q3. Build the SI unit of speed from base units.

    • speed = distance ÷ time
    • metre ÷ second = m s−1

    m s−1

Common mistakes

  • ✗ Multiplying when changing to a bigger unit, e.g. 5000 mm = 5 000 000 m.

    ✓ A bigger unit needs a smaller number: 5000 mm = 5000 ÷ 1000 m = 5 m.

  • ✗ Calling the kilogram a derived unit because it has the prefix kilo.

    ✓ The kilogram is a base unit; it is simply the only base unit with a prefix.

  • ✗ Trying to derive one base unit from another, such as the second from the metre.

    ✓ Base units cannot be derived from one another; only derived units are built from them.

MCQs

  1. 1. MCQ 1.2 (p. 24): One femtometre is equal to:

    1. (a) 10−9 m
    2. (b) 10−15 m
    3. (c) 109 m
    4. (d) 1015 m
    Show answer

    (b) Femto means 10−15 (Table 1.4).

  2. 2. How many base units does SI have?

    1. (a) 5
    2. (b) 6
    3. (c) 7
    4. (d) 9
    Show answer

    (c) Table 1.2 lists seven.

  3. 3. The SI unit of intensity of light is:

    1. (a) candela
    2. (b) kelvin
    3. (c) mole
    4. (d) ampere
    Show answer

    (a) Table 1.2: intensity of light, candela (cd).

  4. 4. 3000 g is equal to:

    1. (a) 3 000 000 kg
    2. (b) 300 kg
    3. (c) 30 kg
    4. (d) 3 kg
    Show answer

    (d) 3000 ÷ 1000 = 3 kg, the book's example (p. 10).

  5. 5. Which of these is a derived unit?

    1. (a) metre
    2. (b) newton
    3. (c) second
    4. (d) kilogram
    Show answer

    (b) Newton is in Table 1.3, the derived units.

Quick revision

  • SI (1961) has seven base units: m, kg, s, K, A, cd, mol.
  • Derived units are built from base units: m2, m s−1, N, Pa.
  • Prefixes multiply a unit by a power of ten: 86 400 s = 86.4 ks.

Before you start

What is Class 9 Physics International System of Units?
International System of Units is the open Class 9 Physics lesson for Matric Part 1, Punjab board. It is a step-by-step lesson, not a PDF of the book.
Which boards is this Class 9 Physics lesson for?
The same Punjab textbook: Lahore, Rawalpindi, Gujranwala, Faisalabad, Multan, Sargodha, Bahawalpur, Dera Ghazi Khan, Sahiwal.
Can I study this in Urdu?
Yes. English, Urdu or Roman Urdu. Science terms stay as the exam writes them.
Is this a notes PDF?
No. The book stays with you. This page is the lesson and the check.

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