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 (empirical) but Newton discovered the underlying law (theoretical). 

Key Concepts

Gravitational attraction can provide the centripetal force necessary to hold a body in a circular orbit.

Equating the equations for gravity and centripetal force leads to Kepler's third law

\(T^2\over r^3\) = constant


Earth satellites can have many different orbits but all must rotate about the centre. Low orbits have the shortest time period.

Test Yourself

Use flashcards to practise your recall.

Exam-style Questions

Online tutorials to help you solve original problems

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