Many students confuse a trajecory in space with a path in a [br]{time, position} plane. [br][br]A trajectory in space is series of images perceived by the eye.[br]A moving point in a {time, position} plane is an image for the mind's eye.[br][br]Consider a line of cars waiting for a traffic light. The light turns green[br]and the cars begin to move.[br][br]What moves forward? How fast? What moves backward? How fast?