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By the end of this topic, you should be able to:
In physics, we measure many different things — from the mass of a book to the speed of light. Every measurement needs a unit to make sense. For example, saying "the table is 2" doesn't tell us much, but "the table is 2 meters long" gives us useful information.
The SI system (International System of Units) is the standard system used by scientists worldwide. It is based on seven fundamental measurements called base quantities. These base quantities cannot be defined using other physical quantities — they are the building blocks from which all other units are made.
For this course, you need to know five SI base quantities:
| Base Quantity | SI Base Unit | Symbol |
|---|---|---|
| Mass | kilogram | kg |
| Length | metre | m |
| Time | second | s |
| Electric Current | ampere | A |
| Temperature | kelvin | K |
Think of these as the "alphabet" of physics measurements. Just like you can make any word from letters, you can make any physics unit from these five base units.
Why are these important?
Most quantities we measure in physics are not base quantities. Instead, they are derived units — units that are made by combining two or more base units together through multiplication or division.
A derived unit is any unit that can be expressed using the SI base units. For example:
To find the base units of any derived quantity, you need to:
Let's look at some important examples:
Force is defined by the equation: Force = mass × acceleration
Step 1: Write the equation F = m × a
Step 2: Replace with units
Step 3: Combine the units Force = kg × m s⁻² Force = kg m s⁻²
So 1 newton (N) = 1 kg m s⁻²
Energy is defined by the equation: Energy = force × distance
Step 1: E = F × d
Step 2: Replace with units
Step 3: Combine Energy = (kg m s⁻²) × m Energy = kg m² s⁻²
So 1 joule (J) = 1 kg m² s⁻²
Pressure is defined as: Pressure = force ÷ area
Step 1: P = F ÷ A
Step 2: Replace with units
Step 3: Combine Pressure = (kg m s⁻²) ÷ m² Pressure = kg m⁻¹ s⁻²
So 1 pascal (Pa) = 1 kg m⁻¹ s⁻²
Power is defined as: Power = work done ÷ time
Step 1: P = W ÷ t
Step 2: Replace with units
Step 3: Combine Power = (kg m² s⁻²) ÷ s Power = kg m² s⁻³
So 1 watt (W) = 1 kg m² s⁻³
Charge is defined by: Charge = current × time
Step 1: Q = I × t
Step 2: Replace with units
Step 3: Combine Charge = A s
So 1 coulomb (C) = 1 A s
Voltage is defined as: Voltage = work done ÷ charge
Step 1: V = W ÷ Q
Step 2: Replace with units
Step 3: Combine Voltage = (kg m² s⁻²) ÷ (A s) Voltage = kg m² s⁻³ A⁻¹
So 1 volt (V) = 1 kg m² s⁻³ A⁻¹
| Quantity | Symbol | Derived Unit Name | Base Units |
|---|---|---|---|
| Force | F | newton (N) | kg m s⁻² |
| Energy/Work | E/W | joule (J) | kg m² s⁻² |
| Pressure | P | pascal (Pa) | kg m⁻¹ s⁻² |
| Power | P | watt (W) | kg m² s⁻³ |
| Charge | Q | coulomb (C) | A s |
| Voltage | V | volt (V) | kg m² s⁻³ A⁻¹ |
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