8.2.5Space Physics
Orbital Speed and Radius — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 10
Section 1 of 6Orbital speed and radius (HT only)
Orbital speed
- Orbital speed (HT only) Orbital speed is the speed at which an object travels while orbiting another object.
Orbital radius
- Orbital radius (HT only) Orbital radius is the distance between the orbiting object and the centre of the object it orbits.
A matched pair
- A matched pair (HT only) For a given separation, the gravitational force gives the object one particular speed that keeps the orbit stable and circular.
Section 2 of 6Distance, force and speed
1Greater radius
- Greater radius (HT only) The orbiting object is further from the centre of the larger body.
2Weaker gravity
- Weaker gravity The gravitational force, which provides the centripetal force, decreases as the separation increases.
3Lower orbital speed
- Lower orbital speed Objects orbiting further away travel more slowly.
4Longer orbit time
- Longer orbit time A larger radius and a lower speed mean a longer time to complete one revolution.
- This is why outer planets take far longer to orbit the Sun.
Section 3 of 6What if the speed is wrong? (HT only)
| Situation | What happens | Reason |
|---|---|---|
| Speed too fast for the radius | The orbital radius increases and the object moves away, possibly out into space. | Gravity is not strong enough to give the needed centripetal force. |
| Speed too slow for the radius | The orbital radius decreases and the object is pulled towards the body it orbits. | Gravity is too strong for the speed, so it pulls the object inwards. |
Section 4 of 6Changing the radius of an orbit
1
(HT only) An orbit is stable only if the speed matches the radius.
2
To move an artificial satellite or moon into a different orbit, the speed at which it travels must change.
3
If the speed increases, the radius of the stable orbit must decrease.
4
If the speed decreases, the radius of the stable orbit must increase.
5
A satellite in a higher orbit therefore travels more slowly than one in a lower orbit, and takes longer for each revolution.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Orbital speed | The speed of an object as it moves in its orbit. |
| Orbital radius | The distance from the centre of the orbit to the orbiting object. |
| Stable orbit | An orbit of constant radius in which the speed matches the gravitational pull. |
| Revolution | One complete trip around an orbit. |
| Centripetal force | The inward force, provided by gravity, that keeps the object on a curved path. |
Section 6 of 6Exam technique
1
Thinking that satellites further out move faster. The further out the orbit, the lower the orbital speed (HT only).
2
Saying a satellite that is too slow flies off into space. Too slow and it spirals inwards; too fast and it moves outwards (HT only).
3
Changing the speed and expecting the radius to stay the same. A different speed means a different stable radius (HT only).
4
Forgetting that a bigger radius also means a longer path, so the time for one revolution is much longer.
5
Measuring the orbital radius from the surface of the planet. It is measured from the centre of the object being orbited.
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