Introduction
Xcel Learning Notes
The International System of Units (SI)
The International System of Units, commonly known as SI, is the standard system used by scientists and professionals worldwide to ensure consistency and easier collaboration.
• Established in 1960 at an international convention in Paris, France.
• It is based on the metric system, which utilizes a decimal system (powers of 10).
• Most countries use the SI system, though some still use the English (Imperial) system.
• Imperial units (non-SI) include miles, inches, and pints.
📌 Exam Tip
CSEC exams primarily use SI units. Always check if your final answer requires a specific unit conversion like cm to m.
Quantities and Units
To understand measurement, we must distinguish between what we are measuring and what we use to measure it.
• A quantity is a physical property that can be measured (e.g., the length of a field).
• A unit is a specific magnitude used as a standard to measure that quantity (e.g., meters or feet).
• General Rule: (Value) × (Unit) = Measurement (e.g., 5 meters).
⚠ Common Mistake
Confusing the quantity with the unit. Length is the quantity; meter is the unit.
Base Quantities
Base quantities (also known as fundamental quantities) are the building blocks of all measurement. They cannot be defined in terms of other physical quantities.
There are five primary base quantities often remembered by the acronym MELTT:
• Mass (measured in kilograms, kg)
• Electric Current (measured in Amperes, A)
• Length (measured in meters, m)
• Temperature (measured in Kelvin, K)
• Time (measured in seconds, s)
📌 Exam Tip
Memorize the five base quantities (MELTT) as they are the foundation for all derived units in Physics and Mathematics.
Derived Quantities
Derived quantities are created by mathematically combining base quantities through multiplication or division.
• Unlike base quantities, they are not fundamental; they depend on the base units.
• Example 1: Area is derived by multiplying Length × Length (m × m = m²).
• Example 2: Speed is derived by dividing Length (distance) by Time (m ÷ s = m/s).
• Example 3: Volume is derived by multiplying Length × Length × Length (m × m × m = m³).
Key Formula
Speed = Distance ÷ Time
Area = Length × Width
Volume = Length × Width × Height
Area = Length × Width
Volume = Length × Width × Height
✏️ Practice Questions
- Define the term ‘Derived Quantity’ and provide two examples.
- Identify the five base quantities used in the SI system.
- State the difference between the Metric system and the Imperial system.
- If a car travels 100 meters in 5 seconds, identify the base quantities used and the derived quantity being calculated.
Xcel Learning Notes — xcellearning.com
