Orbital Speed & Radius (Oxford AQA IGCSE Physics)

Revision Note

Orbital Speed

  • 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

Planets closer to the Sun take a shorter amount of time to orbit and therefore travel at a faster orbital speed than those further away, for IGCSE & GCSE Physics revision notes
The orbit of different planets around the Sun

Orbital Radius

  • 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

A satellite needs to travel at the correct speed to reach a circular orbit. If it travels too fast then it will not maintain its orbit. If it travels too slow then it will not reach a circular orbit
Diagram showing how the speed of an artificial satellite affects its 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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