Gravity

Gravity is a force of nature which causes all objects of mass to attract each other. Gravity holds the planets of star systems together and basically holds the whole universe together.

Sir Isaac Newton developed the theory of gravity. The 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 when all the stars had burned out, planets were found through a graviational 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
Most spaceships had artificial gravity systems. There were also attempts to control or manipulate gravity on a larger scale.

The Gravitron was installed in the moonbase in the year 2050. By using it to alter the Earth's tides, people were able to control the Earth's weather. In 2070 the Cybermen attempted to take over the Gravitron as part of their invasion of Earth. (DW: The Moonbase)

The Tractators could control gravity with their minds. (DW: Frontios)