Topic 1 · 11.1–11.2 · Free
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Chapter 11 — Chapter 11 — Special Theory of Relativity
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📘Easy summary
Motion is always described relative to some reference. A frame of reference is a coordinate system relative to which measurements of position, velocity and time are made. An inertial frame is one that is at rest or moves with uniform velocity (non-accelerated); Newton's first law holds in it (e.g. a train moving at constant velocity; the Earth approximately). A non-inertial frame is accelerated or rotating; fictitious forces appear (e.g. an accelerating car, a merry-go-round, a lift starting). For sound, an observer moving in the same direction as the wave measures a smaller wave speed (relative velocity).
⭐Key points
- Frame of reference = coordinate system.
- Inertial: uniform velocity, Newton's laws hold.
- Non-inertial: accelerated.
🔬Diagram
🌿Real-life example
A ball dropped in a uniformly moving train falls straight down for a passenger.
💡Exam tip
- Earth is approximately inertial for short-duration experiments.
✅Quick check
1. Inertial frame moves with:
2. Observer moving with sound wave measures speed:
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