Lesson 1.1: Quantities, Units & Distance-Time Graphs
Objective: Master the fundamental language of physics and learn to describe basic motion graphically. This lesson is the essential first step.
Part 1: Units & Conversions (8 minutes)
Physics is built on measurement. We use the International System of Units (SI).
Core Idea: Every physical quantity has a standard unit.
- Length is measured in metres (m).
- Time is measured in seconds (s).
- Mass is measured in kilograms (kg).
You Must Know: How to convert between units, especially for speed.
- 1 kilometre (km) = 1000 metres (m)
- 1 hour (h) = 3600 seconds (s)
💡 Quick Example: To convert 72 km/h to m/s:
- Convert km to m:
72 km = 72,000 m - Convert h to s:
1 h = 3600 s - Divide:
72,000 m / 3600 s = 20 m/s
This skill is non-negotiable for solving every motion problem.
Part 2: Scalars vs. Vectors (7 minutes)
All physical quantities are either scalars or vectors. This is a crucial distinction.
| Scalar Quantities | Vector Quantities |
|---|---|
| Have magnitude (size) only. | Have magnitude AND direction. |
| Examples: • Distance (e.g., 5 m) • Speed (e.g., 10 m/s) • Mass, Time, Energy | Examples: • Displacement (e.g., 5 m North) • Velocity (e.g., 10 m/s upwards) • Force, Acceleration, Momentum |
Why does this matter? For vectors, direction changes everything. A velocity of 10 m/s East is completely different from 10 m/s West, even though the speed is the same. When you add vectors, you must consider their direction, not just their size.