| Astrophysics (Index) | About |
The tidal force is the net difference in gravitational force between two bodies of mass 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 mass, 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. The resulting tidal force tends to stretch the body, e.g., making the ocean rise a bit at the point toward the Moon and also rise at the opposite point on Earth, where the tidal force is smaller given that location is more distant from the Moon. The same force-differences affect solid ground, but with a much smaller displacement.
If rotation or orbit causes the direction of such a tidal force to change over time, the (possibly slight) elasticity of a body allows work to be done changing its shape, dissipating energy through frictional heat (tidal heating), 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). This is a minor source of heat within the Earth but significant for numerous of the solar system's moons. Over time, the bodies can settle into a tidally-locked situation (tidal locking) where the bodies' rotation matches the orbit, i.e., the same face of the body always faces its orbiting partner. Such tidally-locked bodies include many solar system moons, including Earth's.
Tidal forces also affect galaxies (galactic tide refers to the effect of tidal force of one galaxy on another), 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.
Within the strong-field gravity of a black hole, tidal forces are extreme, rising to the point where they destroy anything solid. Spaghettification is a term for the extreme stretching of objects along the line through the black hole's center of mass, drawing it together along planes perpendicular to the line. The phenomenon pulls apart approaching stars.