?Physics Forces and motion Q?Watch
What is the difference between instantaneous velocity of a carriage at a point, and the average velocity of a carriage between two ponts?
taken from Physics A FORCES AND MOTION 14 JAN 2002 (2821)
For your other question, the carriage may slow down and speed up, therefore finding the average speed is different to its instantaneous velocity.
*computing really sucks a heap load of ass*