This HL-only mini-lesson covers Theme A.5 — Galilean & special relativity: inertial reference frames, Einstein's two postulates, the Lorentz factor γ, time dilation and length contraction, and mass–energy.
Work through each screen, answer the questions as you go (some are reasoning, some are calculations) and collect ⭐ stars. Watch for the HL flag on higher-level extensions. Press Start when you're ready.
Galilean relativity works well at everyday speeds: velocities simply add. But it fails near the speed of light. Einstein's special relativity rests on two postulates:
The constancy of c forces space and time themselves to be relative — leading to time dilation and length contraction.
The Lorentz factor γ quantifies relativistic effects. It is always ≥ 1 and grows without limit as v → c:
At everyday speeds v ≪ c, γ ≈ 1 and relativity reduces to ordinary Galilean physics — which is why we never notice it.
To a stationary observer, a moving clock runs slow and a moving object is shorter along its motion:
Its 2.0 s onboard interval is measured on Earth as Δt = 1.25 × 2.0 = 2.5 s.
Its 100 m proper length is measured as L = 100 ÷ 1.25 = 80 m.
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Tap a statement on the left, then its match on the right.
Postulates: laws of physics same in all inertial frames; c is invariant
Lorentz factor: γ = 1/√(1−v²/c²) ≥ 1, → ∞ as v → c
Time dilation: moving clocks run slow: Δt = γΔt₀
Length contraction: moving lengths shrink along motion: L = L₀/γ
Mass–energy: rest energy E₀ = mc²; total energy E = γmc²
That completes Special Relativity for IB Diploma Physics HL. Press Finish to see your score.
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