Gravity

Gravity was, as Leela repeated from the Fourth Doctor's teachings, "the force that stops us from flying us into space". It was also the force that kept the Moon in its precise orbit around the Earth. (AUDIO: Energy of the Daleks) According to the Twelfth Doctor, "everything" had mass, including light, heat and radiation, which meant "everything [had] gravity too". (COMIC: The Eye of Torment)

According to the Fifth Doctor, the gravity of a celestial body was dependent on its mass, not necessarily how big it was. Peri Brown added, "And how fast [it was] spinning and what the core [was] made of." (AUDIO: Red Dawn)

4.6 billion years BC, rocks were pulled towards a Racnoss webstar, creating the Earth. (TV: The Runaway Bride)

The Daleks planned to counteract gravity and make the Moon leave Earth's orbit on 31 January 2025, making the planet's axis shift wildly, causing natural disasters and severe climate changes. (AUDIO: Energy of the Daleks)

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. (TV: The Pirate Planet)

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

During a crisis in midflight, the Tenth Doctor claimed his TARDIS would lock onto the nearest place of gravity and land there. (TV: Voyage of the Damned)

Many celestial bodies had gravity that was significantly lower that of Earth's. On one asteroid, the First Doctor, John and Gillian were able to jump high in the air. (COMIC: The Therovian Quest) Later, on the Moon, they reminded two astronauts who had fallen down a crevasse that because of the low gravity they could escape by jumping. (COMIC: Moon Landing) On another visit to the Moon, the Second Doctor, Ben, Polly and Jamie amused themselves by jumping around in the low gravity until Jamie accidentally injured himself. (TV: The Moonbase) According to the Fifth Doctor, Mars had gravity which was "about a third" of the sorts of planets the human, Peri Brown, was used to. The Doctor warned Peri not to "just go jumping about" inside the tomb of Izdaal, as she was "liable" of hitting her head on the structure's ceiling. (AUDIO: Red Dawn)

Manipulating gravity
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. (TV: The Moonbase)

When a Morestran probe ship tried to leave Zeta Minor with anti-matter minerals onboard, it found that the gravity pull of the planet increased. This as due to the influence of the anti-matter creature, unwilling to let them leave with the material.

After the Fourth Doctor fulfilled his promise to return the anti-matter, the ship was able to leave Zeta Minor. (TV: Planet of Evil)

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. (TV: Frontios)

The Byzantium had artificial gravity. On Alfava Metraxis in the 51st century, the Eleventh Doctor shot a gravity globe floating above him and ordered his companions to jump. This caused them to land on the Byzantium above them. (TV: The Time of Angels / Flesh and Stone) The failure of this gravity caused the Weeping Angels pursuing the Doctor, Amy Pond and River Song to fall into a crack in the universe. (TV: Flesh and Stone)

Professor Alice Dubrovnik worked on the graviton inverter, which was used on the Pollyanna on its mission to circumnavigate the Sun. It could flip gravitational force and turn it back on itself. Without it, the Pollyanna would have been "squished" by the Sun's gravitational force. (COMIC: The Eye of Torment)