Space, Time, and Motion
A.1 Kinematics, A.2 Forces and momentum, A.3 Work, energy and power
This is the mechanics core of the whole course — everything later (circular motion, gravitation, even relativity at HL) borrows its vocabulary from here. The examiners test three things relentlessly: whether you can tell distance from displacement, whether you can keep vectors as vectors (not just plug numbers into ), and whether you actually understand that energy and momentum conservation are two separate laws that don't always both apply. Get those three straight and most Topic A questions become arithmetic.
Overview: The Shape of Mechanics
Three lenses on the same motion
Every mechanics question is really asking one of three things. Kinematics asks where and when — pure description of motion with no reference to cause. Forces and momentum ask why motion changes and what happens when objects interact. Work, energy and power ask how much — how much capacity to do things was transferred, and how fast. Examiners love switching between these mid-question, so the first job in any problem is deciding which lens you're being asked to use.
- Kinematics never mentions force or mass — if a question gives you mass and asks for 'the force required', you've left kinematics behind.
- Forces questions live and die on correct free-body diagrams; a wrong diagram guarantees a wrong answer even with perfect algebra.
- Energy and momentum are both conserved quantities, but under different conditions — confusing when each applies is the single most common Topic A error.
The shape of the chapter
Command terms that decide how you answer
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Calculate | Obtain a numerical answer showing the working/formula used | AO2 | Bare correct answer with no substitution shown often loses the method mark even if right |
| Show that | Derive the given result explicitly, substituting numbers so the examiner can check every line | AO2 | Writing only the final given value earns zero — every intermediate step must appear |
| Determine | Obtain a value using given data, usually via a stated method or formula not directly given | AO3 | Must state which physics principle is being used, not just the arithmetic |
| Sketch | Draw a graph with correct general shape, labelled axes, key intercepts/features — no need for a scale | AO2 | Missing axis labels or wrong curvature (e.g. straight line instead of curve) costs marks even if the trend is right |
| Compare | Give similarities and differences explicitly, not just describe both separately | AO3 | Describing A then B with no linking comparison word loses the comparison mark |
Key point
Overview