### tidal force

(affect of a third body on the gravitational force between two bodies)

The tidal force is the difference in the gravitational force of one body upon another due to the presence of a third nearby body. The tides of the Earth's oceans are explained by the presence of the Moon, the Moon acting as the third body, contributing to the total gravitational force on the affected body, the ocean.

When gravitational force is described as between finite-sized objects as opposed to being treated as idealized (infinitesimally-small) point masses, the forces holding the individual objects together are affected by a tidal force, i.e., the gravity of the other body. The ground beneath my feet, besides being held down by Earth's gravity, is pulled slightly by the Moon.

If rotation or orbit causes the direction of such a tidal force to change over time, the elasticity of a body allows work to be done changing its shape, dissipating energy through frictional heat, which can change the orbit and/or rotation of the bodies (labeled tidal acceleration, or in the cases when the acceleration is slowing, tidal deceleration or tidal braking) Over time, the bodies can settle into a tidal locked situation (tidal locking) where the bodies' rotation matches the orbit, i.e., the same face of the body always faces its orbiting partner. This is the case with the Earth's moon and numerous other moons in the solar system.

Tidal forces also affect galaxies, gas clouds, and other astronomical bodies.

Tidal equilibrium takes place when tidal forces bring bodies into an equilibrium state, e.g., when tidal forces no longer affect the kinematics, which can be the case with tidal locking. For some three-body systems, e.g., a star, its planet, and the planet's moon, an equilibrium state is never reached.

(physics,gravity,tidal)
http://en.wikipedia.org/wiki/Tidal_force
http://en.wikipedia.org/wiki/Tidal_acceleration

Referenced by:
binary star
circumplanetary disk
dynamo
galactic tide