Forces and Motion
Forces and Motion is where MYP physics stops being about definitions and starts being about reasoning with numbers and diagrams together. Every question here rewards the same three moves: pick the right formula, show the units, and explain the physical reason behind the number — examiners give separate marks for each. The topic splits into six connected ideas: what keeps things moving in circles, why weight isn't mass, why falling objects stop accelerating, what a force actually is, how forces combine, and how graphs encode speed and acceleration. Master the links between them and the calculations stop feeling separate.
Overview — The Big Picture
Why this topic hangs together
Everything in this unit traces back to one idea: forces change motion, and is the equation that quantifies it. Circular motion is just Newton's laws applied to something changing direction rather than speed. Weight is a force () so it obeys the same rules as any other force when you're checking whether motion is balanced or unbalanced. Friction and air resistance are the forces that make real-world motion messier than 'ideal' motion — they're why terminal velocity exists at all. And motion graphs are simply the visual record of what all these forces produced.
- Mass is a property of matter; weight is a force caused by gravity acting on that mass — conflating the two costs marks on almost every planet/gravity question in the bank.
- A net (unbalanced) force always produces acceleration — never assume 'moving' means 'force is being applied in the direction of motion'.
- Circular motion at constant speed still involves acceleration, because velocity (a vector) is constantly changing direction.
- Graphs are data, not decoration — gradient and area both carry specific physical meaning that examiners expect you to extract numerically.
The shape of the chapter
Command terms this topic actually uses
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| State | Give a short, precise answer with no explanation or working needed. | AO1 | Write the exact formula or fact — extra words earn nothing but wasted time. |
| Define | Give the precise meaning of a term. | AO1 | Must include the defining relationship (e.g. weight = force of gravity ON a mass), not just a synonym. |
| Calculate | Obtain a numerical answer, showing the relevant working. | AO2 | Formula → substitution → answer with unit — each step usually carries its own mark. |
| Show that | Derive the given result, showing every step explicitly. | AO2 | You MUST show the working even though you're given the answer — a bare final line scores zero. |
| Deduce | Reach a conclusion from the information given, showing reasoning. | AO3 | State the pattern in the data AND the physical relationship it implies. |
| Explain | Give reasons supported by physics, not just a description. | AO2 | Needs a 'because' — a description of what happens without the reason scores half marks at best. |
| Describe | Give a detailed account, e.g. of a graph's shape or a process. | AO1/AO2 | Reference specific sections/values (e.g. 'from 6 s to 10 s the speed is constant') rather than vague trends. |
| Discuss | Present balanced viewpoints or implications (often ethical/environmental). | AO4 | Needs at least one point FOR and one AGAINST/limitation to reach full marks. |
Key point
Overview