Orbital Speed & Radius (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Written by: Ann Howell
Reviewed by: Caroline Carroll
Orbital Speed
Extension Tier only
Orbital speed is the speed at which an object travels whilst in orbit around another object
The centripetal force due to gravity decreases as the separation of the orbiting objects increases
This means objects orbiting further away have lower orbital speeds
Objects with a larger orbital radius take more time to make an orbital revolution
Planets orbiting at different distances from the Sun
Orbital Radius
Extension Tier only
Orbital radius is the separation distance between the object in orbit and the centre of the object it is orbiting
When two masses have a particular separation, the centripetal force results in a particular orbital speed
To maintain a stable orbit, a satellite or planet must move at a particular orbital speed
A satellite needs to travel at a specific speed to maintain a circular orbit at a particular distance from the object
If the speed of the satellite is too fast:
The radius of the orbit will increase and the satellite will spiral into space
This is because the gravitational attraction cannot provide enough force to keep it in orbit
If the speed of the satellite is too slow:
The radius of the orbit will decrease and the satellite will be pulled towards the object it should be orbiting
This is because the gravitational attraction is too strong to maintain a constant orbital radius
Maintaining a stable satellite orbit
If an artificial satellite or a moon is to change the radius at which it is orbiting then the speed at which it is travelling must change
To maintain a stable orbit:
If the speed increases the radius must decrease
If the speed decreases the radius must increase
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