This is an example of acceleration ... a change in the speed or direction
of a moving object.
Whether the acceleration is positive of negative depends only on which
direction you named 'positive' before the ball was launched. Decreasing
speed is not necessarily either positive or negative acceleration.
For example:
-- You toss a rock into the air.
-- On the way up, its speed is decreasing.
-- Then, on the way down, its speed is increasing.
-- But the acceleration is constant throughout.
It's the acceleration of gravity, directed downward.
Even though the speed of the rock changes from decreasing
to increasing, the acceleration hasn't changed.
