Motion in the universe: gravity, orbits and orbital speed – IGCSE Physics exam questions with mark scheme

1Grade 1–34 marks

(a) Put these in order of size, starting with the smallest: galaxy, planet, Universe, star. [1]

(b) Name the galaxy that contains our Solar System. [1]

(d) Describe how the shape of a comet's orbit is different from the orbit of a planet. [1]

(a) planet, star, galaxy, Universe(1)
(b) the Milky Way(1)
(c) gravity / gravitational force (of the Sun)(1)
(d) a comet's orbit is (highly) elliptical / eccentric / a stretched oval, whereas a planet's orbit is (almost) circular accept 'comet's orbit is more elliptical/eccentric'; accept comparison either way round; ignore references to speed or time period(1)
Answer: (a) planet, star, galaxy, Universe (b) Milky Way (c) gravity (d) comet orbit highly elliptical
Examiner tip: In (d) make a comparison: a comet's orbit is much more elliptical than a planet's almost circular orbit.

2Grade 4–54 marks

The International Space Station orbits the Earth at an orbital radius of $6.78 \times 10^{6}$ m. It takes 92.6 minutes to complete one orbit.

(a) Name the force that keeps the space station in orbit around the Earth. [1]

(b) Calculate the orbital speed of the space station in m/s. [3]

(a) gravity / gravitational force / weight (of the space station, due to the Earth) ignore 'centripetal force' alone(1)
(b) converts time: 92.6 × 60 = 5556 s(1)
(b) substitution into $v = \dfrac{2\pi r}{T}$: $v = \dfrac{2\pi \times 6.78 \times 10^{6}}{5556}$(1)
(b) 7700 m/s accept 7670 m/s or 7667 m/s; 4.6 × 10⁵ m/s (time left in minutes) scores 1 mark for the substitution only(1)
Answer: (a) gravity (gravitational force of the Earth) (b) 7700 m/s
Examiner tip: Convert the time period to seconds (× 60) before substituting; leaving it in minutes gives 4.6 × 10⁵ m/s.

3Grade 6–75 marks

The Earth orbits the Sun once every 365 days at an average orbital radius of $1.5 \times 10^{11}$ m.

(a) Calculate the orbital speed of the Earth in m/s. [3]

(b) Mars orbits the Sun at a larger radius than the Earth. Describe two ways in which the orbit of Mars is different from the orbit of the Earth. [2]

(a) converts time period: 365 × 24 × 3600 = $3.15 \times 10^{7}$ s(1)
(a) substitution: $v = \dfrac{2\pi \times 1.5 \times 10^{11}}{3.15 \times 10^{7}}$(1)
(a) $3.0 \times 10^{4}$ m/s accept any value that rounds to 3.0 × 10⁴ m/s, e.g. 29 886 m/s, 29 900 m/s, 30 000 m/s; if T is left in days (v = 2.6 × 10⁹) award the substitution mark only(1)
(b) any two from: Mars has a longer time period / takes longer to orbit the Sun; Mars has a lower orbital speed; Mars travels a greater distance in one orbit / its orbit has a larger circumference; the gravitational force (per kg) from the Sun is weaker at Mars; Mars's orbit is more elliptical / less circular ignore 'Mars is further away' (given in the question) — first difference(1)
(b) second, different, difference from the list above(1)
Answer: (a) $3.0 \times 10^{4}$ m/s (b) e.g. Mars has a longer time period and a lower orbital speed
Examiner tip: In (b) do not just repeat that Mars is further away: give two different features of its orbit, such as a longer time period and a lower speed.

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