Gravity And Orbits

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Bound Orbit

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An orbit in which an object is gravitationally bound to the body it is orbiting. A bound orbit's velocity is less than the escape velocity.

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Center of Mass

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The weighted average location of all the mass in a system of objects. The point in any isolated system that moves according to Newton's first law of motion.

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Centripetal Force

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A force directed toward the center of curvature of an object's curved path.

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Circular Velocity

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The orbital velocity needed to keep an object moving in a circular orbit.

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Escape Velocity

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The minimum velocity needed for an object to achieve a parabolic trajectory and thus permanently leave the gravitational grasp of another mass.

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Free Fall

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The motion of an object when the only force acting on it is gravity.

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Galaxy

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A gravitationally bound system that consists of stars and star clusters, gas, dust, and dark matter; typically greater than 1,000 light-years across and recognizable as a discrete, single object.

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Gravity

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1. The mutually attractive force between massive objects; one of four fundamental forces of nature. 2. An effect arising from the bending of spacetime by massive objects.

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Inverse Square Law

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The rule stating that a quantity or effect diminishes with the square of the distance from the source.

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Lagrangian Equilibrium Points

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Also called simply Lagrangian point. One of five points of equilibrium in a system consisting of two massive objects in nearly circular orbit around a common center of mass. Only two Lagrangian points (L4 and L5) represent stable equilibrium. A third, smaller body located at one of the five points will move in lockstep with the center of mass of the larger bodies.

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Lunar Tides

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A tide on Earth that is due to the differential gravitational pull of the Moon.

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Neap Tides

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An especially weak tide that occurs around the time of the first or third quarter Moon, when the gravitational forces of the Moon and the Sun on Earth are at right angles to each other.

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Orbit

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The path taken by one object moving around another object under the influence of their mutual gravitational or electric attraction.

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Perihelion

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The point in a solar orbit that is closest to the Sun.

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Roche Limit

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The distance at which a planet's tidal forces exceed the self-gravity of a smaller object -- such as a moon, asteroid, or comet -- causing the object to break apart.

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Satellite

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An object in orbit about a more massive body -- for example, a human-made satellite, or a moon of any planet.

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Self-Gravity

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The gravitational attraction among all parts of the same object.

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Solar Tide

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A tide on Earth that is due to the differential gravitational pull of the Sun.

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Spherically Symmetric

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Describing an object whose properties depend only on distance from the object's center, so that the object has the same form viewed from any direction.

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Spin-Orbit Resonance

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A relationship between the orbital and rotation periods of an object such that the ratio of their periods can be expressed by simple integers.

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Spring Tide

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An especially strong tide that occurs around the time of a new or full Moon, when lunar tides and solar tides reinforce each other.

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Tidal Bulge

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Distortion of a body resulting from tidal stresses.

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Tidal Force

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A force caused by the change in the strength of gravity across an object.

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Tidal Locking

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Synchronous rotation of an object caused by internal friction as the object rotates through its tidal bulge.

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Tides

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1. On Earth, the rise and fall of the oceans as Earth rotates through a tidal bulge caused by the Moon and the Sun. See also lunar tide, neap tide, solar tide, and spring tide. 2. The deformation of a mass due to differential gravitational effects of one mass on another because of the extended size of the masses.

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Unbound Orbit

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An orbit in which an object is no longer gravitationally bound to the body it was orbiting. An unbound orbit's velocity is greater than the escape velocity.

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Uniform Circular Motion

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Motion in a circular path at a constant speed.

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Universal Gravitational Constant (G)

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The constant of proportionality in the universal law of gravitation. Value: 6.67 x 10-11 newtons times meters squared per kilogram squared [N m2/kg2 = m3/(kg s2)].

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Universal Law of Gravitation

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The law, formulated by Isaac Newton, stating that the gravitational force between any two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them: Fgrav μ (m1m2/r2).

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Weight

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The gravitational force acting on an object; that is, the force equal to the mass of an object multiplied by the local acceleration due to gravity. In general relativity, the force equal to the mass of an object multiplied by the acceleration of the frame of reference in which the object is observed.