Physical Quantities and Units
Problematic — How to measure and express different physical quantities in science?
How to measure and express different physical quantities in science?
- Know the main physical quantities used in science.
- Be able to associate each quantity with its unit of measurement.
- Understand the importance of units to communicate precise measurements.
Part 1: What is a physical quantity?
A physical quantity is a property that can be measured on an object, a substance, or a phenomenon. For example, you can measure the mass of an object, the length of a table, the volume of a liquid, or the temperature of the air.
A physical quantity is always expressed with a numeric value and a unit.
Without a unit, a measurement has no precise meaning. Writing only “2” is not enough: you must specify 2 kg, 2 m, 2 L, or 2 °C.
Physical quantities help to describe the world in a precise and measurable way. In science, we don't just say an object is “big,” “heavy,” or “hot”: we measure its length, mass, or temperature. A correct measurement is always written with a number and a unit. This precision allows us to compare, share results, and conduct reliable experiments.
Part 2: The main quantities and their units
| Quantity | Definition | Unit(s) | Instrument (examples) |
|---|---|---|---|
| Mass | Measurement of the amount of matter in an object | kilogram (kg), gram (g) | Scale |
| Length | Distance between two points | meter (m), centimeter (cm), millimeter (mm) | Ruler, measuring tape |
| Volume | Space occupied by an object or liquid | liter (L), milliliter (mL), cubic meter (m³) | Graduated cylinder, measuring container |
| Temperature | Measurement of thermal state, that is the “degree of hotness or coldness” | degree Celsius (°C) | Thermometer |
Mass and weight are not the same thing. Mass is measured in kg. Weight is a force related to gravitational attraction and is measured in newtons (N). In 6th grade, we mainly work on mass.
Each physical quantity corresponds to a specific measurement and has its own unit. We do not use the same instruments to measure length, mass, volume, or temperature. Knowing how to match the right quantity, unit, and instrument is essential to perform measurements correctly and avoid confusion.
Part 3: The importance of units and conversions
Units allow us to express a measurement clearly and compare it with another. Two people using the same units can immediately understand the meaning of a measurement.
- 1 kg = 1000 g
- 1 m = 100 cm
- 1 cm = 10 mm
- 1 L = 1000 mL
- 1 m³ = 1000 L
| Key idea | Explanation |
|---|---|
| Communication | Units make measurements understandable by everyone. |
| Clarity | A measurement is always written as: value + unit. |
| Conversions | You can change units to adapt the way a measurement is written. |
Units are essential in science because they give a precise meaning to measurements. They also allow comparison of results and clear communication without ambiguity. Conversions help to switch from one unit to another while keeping the same quantity measured. For example, 1 m and 100 cm represent the same length, just expressed differently.
Physical quantities describe measurable properties like mass, length, volume, or temperature. A scientific measurement is always written with a number and a unit. Each quantity has suitable units and specific instruments for measuring it. Conversions finally allow changing units without changing the quantity itself. Understanding this is essential to measure correctly, compare results, and communicate precisely in science.