acceleration The rate of change of velocity. An acceleration may involve a change of speed, direction of motion, or both. 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. 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. 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. 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. centripetal force The central force that produces centripetal acceleration. circle A curve on which all points are equidistant from the center. circular speed The speed that causes an orbiting body to have a circular orbit rather than an elliptic one. 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. ellipse A closed, elongated curve describing the shape of the orbitthat one body follows about another. elliptical galaxy A galaxy having an ellipsoidal shape and lackingspiral arms. escape velocity The speed that an object must have to achieve a parabolic trajectory and escape from its parent body. force A push or a pull. 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. hyperbola A curved path that does not close on itself. A body moving with a speed greater than escape velocity follows a hyperbola. 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. inertial motion Motion in a straight line at constant speed followed by a body when there are no unbalanced forces acting on it. 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. mass A measure of the amount of matter a body contains. Mass is also a measure of the inertia of a body. 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. 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. parabola A geometric curve followed by a body that moves with a speed exactly equal to escape velocity. spring tide Unusually high, high tide and unusually low, low tide that occur when the tidal forces of the Sun and Moon are aligned. tidal force The differences in gravity in a body being attracted by another body. tides Distortions in a body’s shape resulting from tidal forces. velocity A physical quantity that gives the speed of a body and the direction in which it is moving. weight The gravitational force exerted on a body by the Earth (or another astronomical object).