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📘 Lesson 1 of 19 · Mechanics

📏 Physical Quantities

A physical quantity is anything that can be measured — physics begins by carefully defining what we measure and how, before any equations come into play.

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01 Key Concepts

Physical Quantity

Any property of an object or system that can be measured and expressed with a number and a unit, like mass, length, or time.

Fundamental (Base) Quantities

The small set of independent quantities from which all others are derived: length, mass, time, electric current, temperature, amount of substance, and luminous intensity.

Derived Quantities

Quantities built by combining base quantities, such as speed (length/time) or force (mass x length/time^2).

Magnitude and Unit

A complete measurement always needs both a number (magnitude) and a unit -- '5' alone is meaningless without knowing '5 meters' or '5 seconds'.

Dimensional Analysis

Checking that both sides of an equation have matching combinations of base quantities (dimensions), a useful way to catch errors in physics formulas.

02 Solved Examples

Example 1 Classify 'mass' as a fundamental or derived quantity.
  1. Mass is one of the seven base (fundamental) quantities in the SI system.
Answer: Fundamental quantity
Example 2 Classify 'speed' as a fundamental or derived quantity.
  1. Speed is calculated from two base quantities: length divided by time.
  2. Since it's built from other quantities, it is not fundamental.
Answer: Derived quantity (length/time)
Example 3 Why is '10' not a complete physical measurement on its own?
  1. A physical quantity requires both a magnitude and a unit.
  2. '10' alone doesn't specify what is being measured (10 meters? 10 seconds? 10 kg?).
Answer: Because it is missing a unit, so its meaning is undefined

03 Practice Questions

1Is 'time' a fundamental or derived quantity?
Fundamental
2Is 'force' a fundamental or derived quantity?
Derived
3Is 'temperature' a fundamental or derived quantity?
Fundamental
4What two things does a complete physical measurement always need?
A magnitude (number) and a unit
5Is 'area' a fundamental or derived quantity?
Derived (length x length)

📄 Physical Quantities — Downloadable Worksheet

10 questions with a full answer key. Grab the PDF to print, or try the interactive version in your browser.

🗒 Cheat Sheet ⬇ Download PDF 🖥 Interactive Worksheet