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Edexcel IGCSE Physics revision

Edexcel IGCSE Physics Motion in the universe questions

Revise the syllabus content for Motion in the universe, learn how to approach common exam questions, and study two worked examples with clear diagrams.

Edexcel IGCSE Physics Subtopic 8.b

Motion in the universe syllabus

Questions on motion in the universe can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

  • 8.2 know that: the universe is a large collection of billions of galaxies a galaxy is a large collection of billions of stars our solar system is in the Milky Way galaxy
  • 8.3 understand why gravitational field strength, g, varies and know that it is different on other planets and the Moon from that on the Earth
  • 8.4 explain that gravitational force: causes moons to orbit planets causes the planets to orbit the Sun causes artificial satellites to orbit the Earth causes comets to orbit the Sun
  • 8.5 describe the differences in the orbits of comets, moons and planets
  • 8.6 use the relationship between orbital speed, orbital radius and time period: orbital speed = 2 × π × orbital radius / time period v = 2πr / T

How to answer motion in the universe questions

  1. Read both axes and their scales before taking a gradient or area from a graph.
  2. Choose a relationship that contains the quantity asked for and rearrange it before substituting.
  3. For force questions, state the direction as well as the size of a resultant or acceleration.
  4. Link each stage of an explanation: cause, physical change and observed outcome.

Motion in the universe example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Working With Graphs

Question 1

The table gives data for some of the planets in the solar system.
Planet Gravitational field strength at surface in N/kg Orbital radius in km Orbital speed in km/ s
Mercury 3.7 57.9 × 106 47.4
Venus 8.9 108.2 × 106 35.0
Jupiter 23.1 778.6 × 106 13.1
Saturn 9.0 1433.5 × 106 9.7
Uranus 8.7 2872.5 × 106 6.8
Neptune 12.7 4495.1 × 106 5.4
Plot a bar chart of the gravitational field strength for each planet.
Blank plotting grid provided for a bar chart of gravitational field strength for each planet.

Final answer

One correct bar chart is:

0 5 10 15 20 25 3.7 8.9 23.1 9.0 8.7 12.7 Mercury Venus Jupiter Saturn Uranus Neptune Planet Gravitational field strength at surface (N/kg)

Mark scheme points

  1. M1 Use a suitable linear scale, with more than 50% of the available grid used.
  2. M2 Label the axes with the quantities and the unit: gravitational field strength at the surface in N/kg, and planet.
  3. M3 Plot all six bar heights correctly to the nearest half square.

Explanation

  • A linear vertical scale from 0 to 25 N/kg is suitable and uses most of the plotting area.
  • The horizontal axis identifies each planet, while the vertical axis gives gravitational field strength and its unit, N/kg.
  • Draw one bar for each planet with heights: Mercury 3.7, Venus 8.9, Jupiter 23.1, Saturn 9.0, Uranus 8.7 and Neptune 12.7 N/kg.

Common mistakes

  • Do not use a non-linear scale; equal distances on the axis must represent equal changes in gravitational field strength.
  • Do not choose intervals of 3, because this makes accurate plotting and reading difficult and is specifically rejected.
  • Include the y-axis quantity and unit, not just the numerical scale.
  • Draw bars rather than joining the data points with a line graph.
  • Examiners reported that most candidates plotted the chart well and scored all 3 marks; the common losses were an omitted y-axis label or unit and an inappropriate scale.

Example 2: Using a Diagram

Question 2

This question is about satellites and their orbits.
KALPANA-1 was an artificial satellite used to monitor the weather.
The diagram shows the orbit of the satellite.
Diagram of Earth with a circular satellite orbit shown as a dotted circle; satellite labelled KALPANA-1 with an arrow indicating orbital speed; height above Earth's surface indicated.
KALPANA-1 has an orbital speed of 3.1 km/s and completes one orbit in 24 hours.
Calculate the height of KALPANA-1’s orbit above the Earth’s surface.
[radius of Earth = 6400 km]
height above surface = km

Final answer

T = 24 × 60 × 60 = 86 400 s

3.1 = (2 × π × r) / 86 400

r = (3.1 × 86 400) / (2 × π)
r = 42 628 km

height above surface = 42 628 − 6 400
height above surface ≈ 36 000 km

Mark scheme points

  1. M1 Substitute the values into v = 2πr / T.
  2. M2 Convert 24 hours into seconds: T = 86 400 s.
  3. M3 Rearrange and calculate the orbital radius: r = 42 628 km.
  4. M4 Subtract the Earth's radius to find the height: 42 628 − 6 400 ≈ 36 000 km.

Explanation

The satellite travels one circumference, 2πr, in one orbital period, so its speed is given by v = 2πr/T.

The period must be in seconds because the speed is given in kilometres per second. Therefore, 24 hours is converted to 86 400 seconds. Rearranging gives r = vT/(2π), which is the distance from the centre of the Earth to the satellite.

This is not yet the height above the surface. Subtract the Earth's radius:

height = orbital radius − Earth's radius
height = 42 628 − 6 400
height ≈ 36 000 km

Common mistakes

  • Using 24 directly instead of converting the period to 86 400 s.
  • Forgetting the factor 2π in the circumference of the orbit.
  • Giving the orbital radius, approximately 42 628 km, instead of the height above the Earth's surface.
  • Subtracting the wrong value: the Earth's radius is 6400 km, not its diameter.

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