How does the particle model indicate positive acceleration and how does it indicate negative acceleration?

Explanation:

The diagram has a fairly simple explanation. In the top diagram, the space between the particle is increasing. That means that acceleration is increasing. The bottom diagram shows just the opposite. The particle starts off making large "distances" between where the particle is recorded and then the distances between recordings lessens and the particle is slowing down.

Rule: the greater the "distance" between dot positions, the greater the acceleration, because the speed is large.

Top diagram: increasing distance between dots = larger speed. The distance becomes greater as the particle moves to the right.

Bottom diagram: starts off large and decreases as we move from left to right  = - acceleration.  

half of a circle

explanation:

the arrow and the top of the circle is matching together. it is not a polygon because a polygon would always be the same shape. therefore, an half of a circle is called semicircle so you would find the radius of it.