Space, Time, and Motion
A
Topic A is really one idea wearing five costumes: something is conserved, and you just have to find which quantity. Momentum is conserved in collisions, energy is conserved (or converted) in work-energy problems, angular momentum is conserved when torque vanishes, and even mass-energy is conserved once you stop treating and as separate things. HL bolts on two extensions to the SL mechanics you already know — real bodies rotate (not just translate), and at speeds near the rules for time, length and mass stop being absolute. Every exam question here is testing whether you can pick the right conserved quantity and the right frame before you touch a calculator.
Overview — The Shape of the Topic
What this topic actually tests
Four subtopics, four conserved (or transformed) quantities. SL students see kinematics, forces/momentum and work-energy-power in fairly clean linear form. HL students get the same physics rotated (rigid body mechanics) and boosted to relativistic speeds (special relativity, examined as part of the kinematics strand). The examiners love layering these — a question can start as a simple SUVAT problem and end by asking for angular momentum, or start as a momentum collision and end by asking for the energy released as rest mass.
- Kinematics (A.1): describes motion — displacement, velocity, acceleration, graphs, projectiles — without asking why it happens.
- Forces and momentum (A.2): why motion changes — Newton's laws, impulse, conservation of momentum.
- Work, energy and power (A.3): how much 'oomph' is transferred and how fast.
- Rigid body mechanics (A.4, HL only): the rotational mirror-image of A.2 and A.3 — torque replaces force, moment of inertia replaces mass, angular momentum replaces momentum.
- Special relativity (A.5, HL only, examined within the kinematics/energy strands): what happens to time, length, momentum and energy when is not negligible compared with .
The shape of the chapter
Command terms that decide your marks in this topic
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Calculate | Obtain a numerical answer with working shown | AO2 | Method mark for the correct equation/substitution even if the final numeric answer is wrong; final mark needs correct SF and unit. |
| Determine | Find a numerical or algebraic value using data given or derived | AO2/AO3 | Same as Calculate, but often requires you to first derive an intermediate quantity (e.g. γ) before the final substitution. |
| Show that | Prove a given result using explicit physics reasoning | AO2 | You must substitute to at least one more significant figure than the quoted answer — quoting the rounded target value as your own working scores zero. |
| Derive | Obtain a relationship algebraically from stated starting equations | AO3 | Every algebraic step must be shown; skipping from equation 1 to the final form loses the derivation marks even if correct. |
| Sketch | Draw a graph showing the general shape and key features | AO2 | Axes must be labelled with quantity and unit; intercepts/gradient sign must be qualitatively correct even without a scale. |
| Compare | Give both a similarity and a difference between two things | AO3 | A one-sided answer (only differences, or only similarities) forfeits half the marks even if factually correct. |
| Estimate | Give an approximate value using a stated reasonable assumption | AO2 | You must state your assumption explicitly — an unexplained 'estimate' with no reasoning shown scores no method marks. |
Key point
Overview