Motion in the universe questions

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: Calculating a satellite's height above Earth

June 2020 · Paper 1P · Question 11(b)(i)

Question 1

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.

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

The height of KALPANA-1’s orbit above Earth’s surface is 36,228 km (about 36,000 km to 2 significant figures).

Mark scheme points

1. M1 Substitute the orbital speed, radius and time period into v = 2πr / T.
2. M2 Convert 24 hours into 86 400 seconds.
3. M3 Rearrange the equation and calculate the orbital radius: approximately 42 628 km.
4. M4 Subtract Earth’s radius, 6400 km, to find the height above the surface.

Explanation

The equation gives the orbital radius measured from Earth’s centre. Convert the period to seconds so it matches the speed in km/s, then calculate that radius. Finally, subtract Earth’s radius to obtain the height above its surface.

T = 24 × 60 × 60 = 86 400 s

v = 2πr / T
r = vT / (2π)
  = (3.1 km/s × 86 400 s) / (2π)
  ≈ 42 627.7 km

height = orbital radius − Earth’s radius
       = 42 627.7 km − 6400 km
       ≈ 36 228 km

Common mistakes

• Using 24 as the time period without converting hours to seconds.
• Giving the orbital radius as the answer instead of subtracting Earth’s radius to find the height above the surface.

Example 2: Identifying a star, planet and comet

June 2018 · Paper 1P · Question 1(a)

Question 2

Diagram 1 shows the orbits of two objects.

Select words from the box to add the three missing labels to diagram 1.

comet

planet

solar system

star

Final answer

Label the central object star, the object on the nearly circular orbit planet, and the object on the elongated orbit comet, as shown below.

Mark scheme points

1. M1 Label the central object star. “Sun” is also accepted.
2. M2 Label the object on the nearly circular orbit planet. “Earth” or another named planet is also accepted.
3. M3 Label the object on the elongated orbit comet.

Explanation

Use the objects’ positions and the shapes of their orbits to identify them. Check that:

• The star is the object that both orbiting objects travel around. Its gravitational force keeps them in orbit.
• The planet is on the nearly circular path.
• The comet is on the much more elongated path, so its distance from the star changes greatly.

Common mistakes

• Swapping “planet” and “comet”: identify the elongated orbit before adding the labels.
• Using “solar system” for a single object: a solar system includes a star and the objects orbiting it, not just one object.

Practise Motion in the universe questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.