Accepted Answer
It is possible.Consider an object that is moving forward. If it gets a constant negative acceleration, then its speed will decrease. When it becomes negative, the object will move backwards : it will have changed its direction.
Accepted Answer
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.
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