Space physics joins four ideas: gravity as the force that keeps bodies in orbit, the size and layout of the solar system, how stars release energy, and what the light from distant galaxies tells us about the Universe. Each idea is short. The marks are won by linking them in the right order.
Check the current Cambridge IGCSE Physics 0625 syllabus for your exam year to see exactly which statements are listed under this topic, and which are for the Extended route only.
What should you know before starting?
You should be comfortable with speed, distance and time from motion and graphs, and with the idea that a force changes motion from forces and momentum. The wave idea of wavelength and frequency from wave behaviour and the electromagnetic spectrum helps with redshift. Standard form, such as 1.5 × 1011 m, is used throughout.
An orienting example
An invented satellite orbits Earth in a circle of radius 7.0 × 106 m, taking 5800 s for one orbit. How fast does it travel?
In one orbit it covers the circumference: 2πr = 2 × π × 7.0 × 106 = 4.40 × 107 m.
Speed = distance ÷ time = 4.40 × 107 ÷ 5800 ≈ 7.6 × 103 m/s.
Check the size: about 7.6 km every second. That is fast, but satellites close to Earth do move at this scale, so the answer is plausible. The force keeping it in orbit is gravity, which points towards the centre of Earth at every moment.
In what order should you study the lessons?
- Relate orbital motion to a force direction: start with why a circular orbit needs a force towards the centre, and use speed = circumference ÷ period.
- Interpret a scale diagram of the solar system: turn distances on a drawing into real distances, and see why sizes cannot share the scale.
- Explain a star’s energy source at syllabus level: fusion, and the balance of forces that keeps a stable star steady.
- Use a supplied redshift relationship where applicable: substitute carefully, with units, into a relationship given to you.
- Distinguish observational evidence from a model conclusion: the skill that ties the module together and shapes how you write answers.
What traps catch students in this topic?
- Inventing a forward force. A planet does not need a force pushing it along its path. Gravity acts towards the centre and changes its direction.
- Reading a diagram as if it were to scale. Most solar system pictures squeeze the distances or enlarge the planets.
- Saying stars “burn” hydrogen. A star releases energy by fusion, not by combustion.
- Unit slips in redshift. Nanometres, metres and kilometres per second are easy to mix when a ratio hides them.
- Writing “proves”. An observation is evidence. A model is an explanation that fits it.
How should you use the practice set?
Work through the lessons first, then try the space physics practice set on paper. Open each answer only after a full attempt, and use the table at the end to send each slip back to its lesson. The mistake log and retest queue helps you see which kind of slip keeps returning.
For more of the subject, see the Physics learning guide. Students who follow each step but still lose marks in written explanations can benefit from online one-to-one Physics tuition, where a teacher reads your wording line by line.