Of course ! If you call "up" the "positive" direction, then that's exactly
what happens every time you toss a stone or a ball up into the air.
The stone or the ball starts out with upward (positive) velocity. But the
acceleration of gravity is downward (negative). The effect on the ball is
that its positive velocity becomes smaller and smaller, eventually reaches
zero, and then becomes negative when the ball or the stone starts moving
downward.
