Question

Explain why average velocity in one dimension can be positive or negative.

Answer 1

 One dimensional velocity is based upon a straight line. For example: You can only go forwards or backwards. You assign one direct (lets say forward) to be positive.  So if you move backwards, you are technically moving in a negative direction 

Answer 2

Imagine an object is moving in one dimension on a number line, and for this we'll say that the numbers on the line are a metre apart. If the object moves from 2 m to 7 m, the change in position is 7-2=+5 metres. But if the object moves back from 7 m to 2 m, the change in position is 2-7=-5 metres. since $velocity = \frac{change in position}{time}$, and time is always positive, velocity will be positive in one direction and negative in the other direction.