Gravity

Gravity was a force of nature which caused all objects of mass to attract each other. Gravity caused the planets of star systems to stay in orbit.

Sir Isaac Newton developed the theory of gravity. The Fourth Doctor once claimed that he sat in Newton's tree dropping apples onto his head, then explained gravity to him over dinner. (DW: The Pirate Planet)

In the year 100,000,000,000,000 BC when all the stars had burned out, planets were found through a gravitional field navigational system. (DW: Utopia)

During a crisis in midflight, the TARDIS locked onto the nearest place of gravity and landed there. (DW: Voyage of the Damned)

Manipulating gravity
Many spaceships had artificial gravity systems.

The Gravitron was installed in the moonbase in the year 1054. By using it to alter the Earth's tides, people were able to control the Earth's weather. In 1401 the Cybermen attempted to take over the Gravitron Executive Board as part of their invasion of Earth. (DW: The Moonbase)

The Tractators could control gravity with their minds. This power could be amplified by tunnel systems built in a precise way. On Frontios, the Tractators used an excavation machine to create a series of tunnels with which to create a gravity motor. (DW: Frontios)