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Negative acceleration, slowing down of motion.
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One of the two angles uses to specify location on the celestial sphere. Declination is like latitude, but unlike latitude, it is measured from the north pole. The pole has declination 0, the equator 90 degrees, the southern celestial pole 180 degrees. See right ascension and declination
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A device for efficiently converting the energy of a hot gas to kinetic energy of motion, originally used in some steam turbines and now used in practically all rockets. By constricting the outflow of the gas until it reaches the velocity of sound and then letting it expand again, an extremely fast jet is produced.
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A flat optical surface, transparent or reflecting, ruled with many parallel grooves at precisely spaced distances. The active parts are not the grooves but the flat sections left between them, which act like a large number of precisely spaced slits. The light waves passing those slits resonate with each other in a way which depends on wavelength, causing different wavelengths to be steered in different directions. The overall effect on light containing different wavelengths is like that of a glass prism: the intensity of the light deflected is much smaller than with a prism, but the ability to separate close colors is much better.
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the air resistance encountered by a moving object. Drag is one of the four forces sensed by an airplane, the others being lift, thrust and weight.
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