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Circular motion and gravity

This image shows satellites in orbit around the Earth (not to scale!). Some of them are in low orbits and some in high orbits. 

In this topic we will consider the following:

  • How does the radius of a satellite's orbit affect its speed?
  • How do satellites stay in orbit even though they are not powered by anything?
  • What is the relationship between gravitational potential energy and position for a non-uniform field?


  • Uniform field

    The Earth's field isn't really uniform but it's close enough when we are near the surface. We often make simplifications like this which is OK providing we don't forget the limitations.

  • Centripetal force

    If a body moves in a circle there must be a force acting towards the centre, always. Simple as that. Deriving the equation is not so simple though.

  • Examples of circular motion

    Space men and cowboys, roller coasters and the wall of death. All good stuff.

  • Circular motion

    If your speed is constant how can you be accelerating. When you move in a circle it feels like you are being thrown out so how come the force is towards the centre? If too many people join me on a roller coaster will we all fall to our death

  • Orbits

    Communications satellites, spy satellites, weather satellites, the moon and the planets are all in orbits governed by the same equations. Kepler worked it out from the data but Newton discovered the underlying law. TV satellites are always in the same pl

  • Newton's law of gravitation

    The force between two 1kg masses is very small, it was a big step for Newton to realise that it exists without being able to measure it. Newton´s universal law of gravity. A law that applies to the whole universe is certainly worth remembering. Field c