O Level Physics: Space physics — Practice Questions (Cambridge 5054)

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Most questions practise a skill tested in the June 2025 Paper 22. After each answer there is a tip and, where a paper question matches the skill, the real question to try next.

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Questions

1. (a) List the eight planets of the Solar System in order, starting with the one nearest the Sun. (b) Name the two planets whose orbits are closer to the Sun than the orbit of the Earth. [3]

2. A comet travels around the Sun in a long, stretched elliptical orbit. (a) Name the force that keeps the comet in its orbit. (b) State the direction of this force at any point on the orbit. (c) As the comet travels outwards from its closest point to the Sun to its furthest point, what happens to the size of the Sun’s pull on it? Give a reason. [4]

3. The planet Mars has an average orbital radius of 2.28 × 10⁸ km and takes 687 days to orbit the Sun once. Calculate the average orbital speed of Mars in km/s. [3]

4. Two moons, A and B, orbit the same planet. Moon A has an orbital radius of 3.0 × 10⁵ km and an average orbital speed of 12 km/s. Moon B has an orbital radius of 1.2 × 10⁶ km and an average orbital speed of 6.0 km/s. While moon B goes once round the planet, how many complete orbits does moon A make? Show your working. [3]

5. Describe how a star such as the Sun forms from an interstellar cloud of gas and dust, up to the point at which it becomes a stable star. [3]

6. (a) Describe what is meant by redshift. (b) Light from galaxy P shows a greater redshift than light from galaxy Q. State what this tells you about galaxy P compared with galaxy Q. (c) State how the redshift of distant galaxies provides evidence for the Big Bang theory. [3]

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Answers

1. (a) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune [2] (all eight in the correct order for 2 marks; one mistake for 1 mark). (b) Mercury and Venus [1].

Tip: learn the order with a memory sentence, and remember that Mars lies between the Earth and Jupiter, with the asteroid belt between Mars and Jupiter.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 9.

2. (a) The gravitational attraction of the Sun (gravity) [1]. (b) Towards the Sun (towards its centre) [1]. (c) The force decreases as the comet moves away from the Sun [1], because the strength of the Sun’s gravitational field decreases as the distance from the Sun increases [1].

Tip: the force always points towards the Sun, not along the direction the comet is moving.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 9.

3. T = 687 × 24 × 3600 = 5.94 × 10⁷ s [1]. v = 2πr ÷ T = (2π × 2.28 × 10⁸) ÷ (5.94 × 10⁷) [1] = 24 km/s [1].

Tip: keep the radius in km and the time in seconds, and the speed comes out directly in km/s; the distance round one orbit is 2πr, not r.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 9.

4. Using T = 2πr ÷ v: for moon A, T = (2π × 3.0 × 10⁵) ÷ 12 = 1.57 × 10⁵ s [1]; for moon B, T = (2π × 1.2 × 10⁶) ÷ 6.0 = 1.26 × 10⁶ s [1]. Number of orbits = 1.26 × 10⁶ ÷ 1.57 × 10⁵ = 8 [1].

Tip: rearrange v = 2πr ÷ T to find each period first, then divide the longer period by the shorter one; a whole-number answer is a good check.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 9.

5. The cloud, which contains hydrogen, collapses under its own gravitational attraction to form a protostar [1]. As it collapses its temperature increases [1]. It becomes a stable star when the inward force of gravity is balanced by an outward force due to the high temperature at its centre, where hydrogen is fusing into helium [1].

Tip: use the word “protostar” and give the balance of forces in full: gravity acting inwards against the outward force from the high temperature.

6. (a) Redshift is an increase in the observed wavelength of light (electromagnetic radiation) from a star or galaxy that is moving away from the Earth [1]. (b) Galaxy P is further away and is moving away from the Earth faster than galaxy Q [1]. (c) Light from all distant galaxies is redshifted, so they are all moving apart and the Universe is expanding; going back in time, everything must have started from a single very small point [1].

Tip: redshift means the wavelength gets longer; it does not mean the light turns red.

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Where marks are usually lost

• Mixing up the order of the planets, especially Mercury and Venus or Uranus and Neptune.
• Drawing or describing the gravitational force along the path of the orbit instead of towards the Sun.
• Leaving the orbital period in days when the speed is needed in km/s.
• Forgetting that orbital distance is 2πr when using v = 2πr ÷ T.
• Describing redshift as light “becoming red” rather than an increase in wavelength.