What is in our solar system, and what keeps objects in orbit?
The solar system and orbits: the structure of the solar system, the role of gravity, and why orbital speed and radius are linked for circular orbits.
A focused answer to AQA GCSE Physics 4.8.1, covering the structure of our solar system, how gravity holds planets, moons and satellites in orbit, and why the orbital speed and radius are linked for a stable circular orbit (separate physics).
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What this dot point is asking
AQA wants you to describe the structure of the solar system, explain how gravity keeps objects in orbit, and explain why, for a stable circular orbit, a change in speed requires a change in orbital radius. This is part of topic 4.8.1 of the AQA GCSE Physics (8463) specification and is separate physics only.
The structure of the solar system
The Sun formed when a cloud of dust and gas (a nebula) was pulled together by gravity until it became hot and dense enough for nuclear fusion to begin. The leftover material around the young Sun went on to form the planets, which is why they all orbit in roughly the same plane and direction. Comets, by contrast, follow highly elongated (very stretched) orbits that take them close to the Sun and then far out into the solar system, which is why a comet speeds up as it approaches the Sun and slows down as it moves away: it is moving fastest where it is closest and gravity is strongest.
Gravity and orbits
The size of the gravitational force depends on the masses involved and on the distance between them, getting weaker the further apart the objects are. This is why the planets closest to the Sun, such as Mercury, feel the strongest pull and must orbit fastest, while distant planets such as Neptune feel a weaker pull and orbit slowly. Artificial satellites are a useful everyday example: communication and weather satellites are deliberately placed at the orbital radius that matches the speed needed to stay in a stable orbit, and a geostationary satellite is placed at exactly the radius where its orbital period is one day, so it appears to stay fixed above one point on the Earth.
Circular orbits
Try this
Q1. State what force keeps a planet in orbit around the Sun. [1 mark]
- Cue. Gravity (the gravitational force of the Sun).
Q2. Explain why an object in a circular orbit is accelerating even though its speed is constant. [2 marks]
- Cue. Gravity continually changes the direction of motion, so the velocity changes; a changing velocity means it is accelerating.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA satellite moves in a stable circular orbit around the Earth. Explain why the satellite is accelerating even though its speed is constant, and explain the direction of the force that causes this acceleration.Show worked answer →
The satellite is accelerating because acceleration means a change in velocity, and velocity is a vector that includes direction (1 mark). Although the speed (the size of the velocity) stays constant, the direction of motion is continuously changing as the satellite goes round its circular path, so the velocity is constantly changing, which is an acceleration (1 mark). The force causing this acceleration is gravity, the gravitational attraction of the Earth on the satellite (1 mark), and this force acts towards the centre of the orbit (towards the centre of the Earth), continually pulling the satellite inwards and changing its direction (1 mark). Markers reward the changing-direction-means-changing-velocity reasoning and gravity acting towards the centre.
AQA 20213 marksExplain why, for an object in a stable orbit, a change in the speed of the object requires a change in the radius of its orbit.Show worked answer →
For a stable orbit at a particular radius, there is only one speed at which the gravitational force exactly provides the force needed to keep the object moving in that circle (1 mark). If the object moves faster than this, the gravitational force is no longer enough to hold it at that radius, so it moves to a larger orbital radius; if it moves slower, gravity pulls it into a smaller orbital radius (1 mark). So a stable orbit links one speed to one radius: to orbit stably at a different speed, the object must orbit at a different radius, with faster orbits at smaller radii and slower orbits at larger radii (1 mark). Markers reward the idea of a single speed for each radius and the correct direction of the speed-radius relationship.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)