AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
(HT only) Students should be able to explain qualitatively how: (HT only)…
Full specification wording
Orbital motion, natural and artificial satellites: (HT only) Students should be able to explain qualitatively how: (HT only) for circular orbits, the force of gravity can lead to changing velocity but unchanged speed (HT only) for a stable orbit, the radius must change if the speed changes.
Speed is a scalar and velocity is a vector, so velocity carries a direction with it.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Speed is a scalar and velocity is a vector, so velocity carries a direction with it. In a circular orbit gravity pulls the satellite towards the centre of the circle, at right angles to the way it is moving. A force at right angles to the motion changes the direction of travel without changing how fast the object is going, so the satellite is accelerating all the way round and its velocity is changing all the way round, yet its speed stays constant. The second idea is about staying in orbit. Gravity is stronger closer in, so a satellite in a low orbit has to travel faster to stay there. Speed and radius are therefore locked together: if the speed increases the radius must decrease, and if the speed decreases the radius must increase.
What earns the marks
- The gravitational force acting towards the centre, at right angles to the motion.
- The direction of motion changing, so that the velocity changes.
- The speed staying the same, because the force has no component along the direction of motion.
- A faster satellite needing a smaller orbital radius if the orbit is to be stable.
- A slower satellite needing a larger orbital radius.
Common mistakes and how to put them right
Treating speed and velocity as the same quantity, and so concluding that the satellite speeds up.
Saying there is no acceleration because the speed is constant.
Stating that a faster satellite orbits further out.
Saying that the radius must change without ever saying which way for a given change in speed.
Exam tips
- Open with the difference between a vector and a scalar; it is usually the first mark in these questions.
- Put changes direction and unchanged speed in the same sentence so that both ideas are clearly separate.
- Quote a real pair, the Moon and the International Space Station, to show that a lower orbit means a faster satellite.
Try the lesson
The start of the Orbital motion, natural and artificial satellites: (HT only) Students should be able to explain qualitatively how: (HT only) for circular orbits, the force of gravity can lead to changing velocity but unchanged speed (HT only) for a stable orbit, the radius must change if the speed changes. lesson, made from checked teaching notes for this specification point.
Have a go first
For a satellite in a circular orbit, why does the force of gravity change its velocity but not its speed?
Listen
Speed is a scalar and velocity is a vector, so velocity carries a direction with it. In a circular orbit gravity pulls the satellite towards the centre of the circle, at right angles to the way it is moving. A force at right angles to the motion changes the direction of travel without changing how fast the object is going, so the satellite is accelerating all the way round and its velocity is changing all the way round, yet its speed stays constant. The second idea is about staying in orbit. Gravity is stronger closer in, so a satellite in a low orbit has to travel faster to stay there. Speed and radius are therefore locked together: if the speed increases the radius must decrease, and if the speed decreases the radius must increase. A strong answer earns marks for these points. the gravitational force acting towards the centre, at right angles to the motion. the direction of motion changing, so that the velocity changes. the speed staying the same, because the force has no component along the direction of motion. a faster satellite needing a smaller orbital radius if the orbit is to be stable. Exam tip: Open with the difference between a vector and a scalar; it is usually the first mark in these questions.
Key words
Say what each word means before you turn the card.
scalar and vector
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gravity acting towards the centre
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constant speed with changing velocity
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stable orbits
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See every definition
- scalar and vector
- Speed is a scalar and velocity is a vector, so velocity carries a direction with it.
- gravity acting towards the centre
- In a circular orbit gravity pulls the satellite towards the centre of the circle, at right angles to the way it is moving.
- constant speed with changing velocity
- The satellite is accelerating all the way round and its velocity is changing all the way round, yet its speed stays constant.
- stable orbits
- Gravity is stronger closer in, so a satellite in a low orbit has to travel faster to stay there, and if the speed increases the radius must decrease.
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