•Spaces that obey the Euclidean rules of geometry are termed flat. •Zero curvature – the Euclidean case where parallel lines stay parallel•Positive curvature – exemplified by a sphere where parallel lines cross each other•Negative curvature – exemplified by a saddle where parallel lines stray away from each other on an object rather than cross each other. •Geodesic – the shortest distance path between two points. (straight line in Euclidean space but NOT in a curved space)•Equivalence principle – the premise that no experiment can distinguish between gravity and acceleration in a small closed room •Inertia –the tendency of an object to maintain an initial state of motion and resist any attempt to change that state. (i.e. stopping a train going 100 mph)•Einstein’s theory of general relativity – characterizes the effect of gravity in terms of ordinary inertia, acting in a region of space that is curved due to the presence of matter and energy.
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