| acceleration | The rate of change of velocity. An acceleration may involve a change of speed, direction of motion, or both.
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| acceleration of gravity | The acceleration of a body, equal to 9.8 metersper second per second, caused by the force of gravity near the surface of the Earth.
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| angular momentum | The momentum of a body associated with its rotation or revolution. For a body in a circular orbit, angular momentum is the product of orbital distance, orbital speed, and mass.
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| central force | A force directed at the center of motion of a body. Gravity is the central force that accounts for the orbital motion of solar system bodies.
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| centripetal acceleration | The acceleration toward the center of motion, that causes the path of an orbiting body to continually bend away from a straight-line path.
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| centripetal force | The central force that produces centripetal acceleration.
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| circle | A curve on which all points are equidistant from the center.
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| circular speed | The speed that causes an orbiting body to have a circular orbit rather than an elliptic one.
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| conic section | One of four kinds of curves (circle, ellipse, hyperbola, and parabola) that can be formed by slicing a right circular cone with a plane.
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| ellipse | A closed, elongated curve describing the shape of the orbitthat one body follows about another.
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| elliptical galaxy | A galaxy having an ellipsoidal shape and lackingspiral arms.
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| escape velocity | The speed that an object must have to achieve a parabolic trajectory and escape from its parent body.
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| force | A push or a pull.
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| gravitational potential energy | The energy stored in a body subject to the gravitational attraction of another body. As the body falls, its gravitational potential energy decreases and is converted into kinetic energy.
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| hyperbola | A curved path that does not close on itself. A body moving with a speed greater than escape velocity follows a hyperbola.
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| inertia | The tendency of a body at rest to remain at rest and a body in motion to remain in motion at a constant speed and in constant direction.
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| inertial motion | Motion in a straight line at constant speed followed by a body when there are no unbalanced forces acting on it.
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| kinetic energy | Energy of motion. Kinetic energy is given by one half the product of a body’s mass and the square of its speed.
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| mass | A measure of the amount of matter a body contains. Mass is also a measure of the inertia of a body.
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| momentum | A quantity, equal to the product of a body’s mass and velocity, used to describe the motion of the body. When two bodies collide or otherwise interact, the sum of their momenta is conserved.
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| neap tide | An unusually low high tide and unusually high low tide that occur when the tidal forces of the Sun and Moon act at right angles to one another.
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| parabola | A geometric curve followed by a body that moves with a speed exactly equal to escape velocity.
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| spring tide | Unusually high, high tide and unusually low, low tide that occur when the tidal forces of the Sun and Moon are aligned.
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| tidal force | The differences in gravity in a body being attracted by another body.
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| tides | Distortions in a body’s shape resulting from tidal forces.
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| velocity | A physical quantity that gives the speed of a body and the direction in which it is moving.
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| weight | The gravitational force exerted on a body by the Earth (or another astronomical object).
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