Suppose astronomers discover a radio message from a civilization whose planet orbits a star 35 light-years away. their message encourages us to send a radio answer, which we decide to do. suppose our governing bodies take 2 years to decide whether and how to answer. when our answer arrives there, their governing bodies also take two of our years to frame an answer to us. how long after we get their first message can we hope to get their reply to ours? (a question for further thinking: once communication gets going, should we continue to wait for a reply before we send the next message? )

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Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.