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angular momentum
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the measure of the motion of a rotating object in terms of its speed and how widely the object’s mass is distributed around its axis
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aphelion
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the point in its orbit where a planet (or other orbiting object) is farthest from the Sun
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apogee
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the point in its orbit where an Earth satellite is farthest from Earth
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asteroid belt
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the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun
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astronomical unit (AU)
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the unit of length defined as the average distance between Earth and the Sun; this distance is about 1.5 × 10
8
kilometers
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density
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the ratio of the mass of an object to its volume
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eccentricity
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in an ellipse, the ratio of the distance between the foci to the major axis
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ellipse
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a closed curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same
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escape speed
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the speed a body must achieve to break away from the gravity of another body
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focus
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(plural: foci) one of two fixed points inside an ellipse from which the sum of the distances to any point on the ellipse is constant
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gravity
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the mutual attraction of material bodies or particles
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Kepler’s first law
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each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse
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Kepler’s second law
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the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time
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Kepler’s third law
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the square of a planet’s orbital period is directly proportional to the cube of the semimajor axis of its orbit
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major axis
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the maximum diameter of an ellipse
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mass
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a measure of the amount of material within an object
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momentum
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the measure of the amount of motion of a body; the momentum of a body is the product of its mass and velocity; in the absence of an unbalanced force, momentum is conserved
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Newton’s first law
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every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force
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Newton’s second law
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the change of motion of a body is proportional to and in the direction of the force acting on it
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Newton’s third law
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for every action there is an equal and opposite reaction (
or:
the mutual actions of two bodies upon each other are always equal and act in opposite directions)
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orbit
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the path of an object that is in revolution about another object or point
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orbital period (
P
)
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the time it takes an object to travel once around the Sun
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orbital speed
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the speed at which an object (usually a planet) orbits around the mass of another object; in the case of a planet, the speed at which each planet moves along its ellipse
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perigee
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the point in its orbit where an Earth satellite is closest to Earth
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perihelion
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the point in its orbit where a planet (or other orbiting object) is nearest to the Sun
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perturbation
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a small disturbing effect on the motion or orbit of a body produced by a third body
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satellite
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an object that revolves around a planet
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semimajor axis
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half of the major axis of a conic section, such as an ellipse
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velocity
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the speed and direction a body is moving—for example, 44 kilometers per second toward the north galactic pole