In motion along a single straight line there are only two directions, right and left. So instead of drawing two kinds of arrow, we can let the sign of a number carry the direction.
Take rightward as positive. Then a body moving right has a positive velocity and a body moving left has a negative one. Position is measured under the same agreement, positive to the right of the origin and negative to the left.
With that agreement in place, every statement about direction turns into an addition or a subtraction. This is why the single equation serves rightward and leftward motion alike.
As long as the velocity and the acceleration point the same way, the speed keeps growing. The acceleration is pushing the velocity along.
The lower lane is the one to watch. There the velocity and the acceleration are both negative, which is to say both point left. Their directions still agree, so the body still speeds up. The stamped dots spread out in either lane.
A negative acceleration does not mean slowing down. What decides whether a body slows is not the sign of the acceleration by itself, but how that sign pairs with the sign of the velocity.
Now give a body that is running to the right an acceleration pointing left. The velocity is positive and the acceleration negative, so the signs disagree. The speed falls.
The stamped dots crowd together and the line on the - graph closes in on zero. The acceleration itself, however, has not changed at all. The slope is the same from beginning to end. It is the velocity that is changing.
The word deceleration is only a name we hang on this situation. As far as the body is concerned, it is simply being given an acceleration that points against its motion.
Keep the opposing acceleration on and the velocity reaches zero. In the figure, the velocity arrow is the one that vanishes.
The acceleration arrow does not. Its length and its direction are exactly as they were. So the body does not stay stopped there: the next moment it picks up a velocity in the other direction and starts back. A ball thrown straight up seems to hang at the top and yet comes down for this very reason.
and are different statements. A velocity of zero says only that the body is not moving at this instant, and that is no reason for it to stay that way.
Look at the graph and the line simply passes through zero. It neither bends nor breaks. The turn is not a moment when something inside the motion switches over. It is one continuous motion, and the turn is only the point where we start calling the direction by another name.