AQA GCSE Physics Foundation (8463), Foundation tier · Space physics › Solar system, orbits and satellites
Practise Orbital motion and satellites. 6 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How gravity keeps planets, moons and artificial satellites in their orbits, and how these objects are similar and different. GCSE Physics only, not in Combined Science.
Key facts
Gravity provides the force that keeps planets and satellites in their orbits
Planet: orbits a star. Moon: a natural satellite that orbits a planet
Artificial satellite: made by people and launched into orbit
One circular orbit: distance = 2 × π × radius
Speed = distance ÷ time (time in seconds for m/s)
Notes
Gravity and orbits
Gravity is the force that keeps planets, moons and artificial satellites moving in their orbits.
You can treat these orbits as circles.
Without gravity, an orbiting object would carry on moving in a straight line and fly off into space.
Planets, moons and artificial satellites
A planet orbits a star (in our solar system, the Sun).
A moon is a natural satellite: it orbits a planet.
An artificial satellite is made and launched by people. Most orbit the Earth, for jobs such as communications, weather monitoring and navigation.
Similarities: all are kept in orbit by gravity, all orbit a more massive object, and all move in (nearly) circular orbits.
Differences: what they orbit, whether they are natural or made by people, their size, and the time they take to complete one orbit.
How to answer each type of question
Give a similarity or a difference between orbiting objects
1 to 2 marksGrade 4
Say what each object orbits: a star, a planet, or (for most artificial satellites) the Earth.
Say whether each object is natural or made by people.
For a similarity use gravity or the circular orbit. For a difference, write about both objects.
Example. The Moon and a weather satellite both orbit the Earth. Give one similarity and one difference between the Moon and the weather satellite.
Show the model answerHide the model answer
Similarity: both are kept in orbit by the Earth's gravity (1). Difference: the Moon is a natural satellite, whereas the weather satellite was made and launched by people (1). Also accept for the difference: the Moon is much bigger, or the Moon takes longer to complete one orbit.
Don’t lose marks
Saying there is no gravity in space, or no force on a satellite. Gravity is what keeps it in orbit.
Saying the resultant force acts outwards or in the direction of motion. It is gravity, acting towards the centre.
Leaving the time in minutes or hours in a speed calculation. Convert to seconds to get m/s.
Using a height above the Earth's surface as the orbit radius. The radius is measured from the centre of the Earth, so add the Earth's radius if you are given a height.
Mixing up the objects: a moon orbits a planet, not the Sun; an artificial satellite is not a moon.
More tips
Memory tricks
Imagine whirling a ball on a string: the string pulls the ball inwards, like gravity. If the string breaks, the ball flies off in a straight line.
Sense check: satellites orbiting the Earth usually travel at a few thousand metres per second. A much smaller answer usually means the time was left in minutes or hours.
Exam technique
For similarity and difference questions, write a full comparison, e.g. 'A moon orbits a planet, whereas a planet orbits a star.'
In orbit calculations, show each step (distance, time in seconds, speed) and give the unit.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State the force that keeps objects in orbitGravity provides the force that keeps planets, moons and artificial satellites in their orbits.
Grade 4
Tell apart planets, moons and artificial satellitesPlanets orbit a star, moons orbit planets, and artificial satellites are made and launched by people.
Grade 5
Give similarities between orbiting objectsAll are held in orbit by the gravity of a more massive object and move in (nearly) circular orbits.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The Moon is a natural satellite. Give one example of an artificial satellite.
A weather satellite (or any other named artificial satellite).
Which force keeps a satellite moving in a circular orbit around the Earth?
Gravity.
Give one use of a geostationary satellite.
Satellite television broadcasting (or similar).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
The Moon orbits the Earth. The Earth orbits the Sun.
(a) Which force keeps the Moon in its orbit around the Earth? Tick (✓) one box.[1]
Friction
Gravity
Magnetism
Tension
(b) The Moon is a natural satellite. Give one example of an artificial satellite.[1]
(c) What shape is the orbit of the Moon, approximately?[1]
(d) Give one use of artificial satellites.[1]
Show the answer and mark scheme
(a)Answer: Gravity
(b)Answer: A weather satellite (or any other named artificial satellite).
the International Space Station / a communications, weather, navigation (GPS) or observation satellite / a space telescope
(c)Answer: Circular
circular
(d)Answer: Communications (or weather forecasting, navigation, observing the Earth).