Final answer:
Objects moving first in the positive direction and then in the negative experience motion with constant acceleration, where initially positive velocity is reduced by a negative acceleration, such as gravity, leading to a reversal of direction.
Step-by-step explanation:
When an object first moves in the positive direction and then moves in the negative direction, this can be an instance of motion with constant acceleration. For example, consider an object thrown straight up into the air. Initially, its velocity is positive as it moves upward. When it reaches the maximum height, its velocity becomes zero for an instant, and then, as gravity acts upon it, the velocity becomes negative while it falls back down. This is because it is moving back towards the starting point. Throughout this process, the acceleration due to gravity points downward, thus it is constantly negative, causing the object to slow down as it ascends and to speed up as it descends.
Velocity and acceleration are two key concepts in these scenarios. If we describe positive direction as upward or to the right, the initial positive velocity decreases due to the negative acceleration (e.g., gravity), bringing the object to a stop before it reverses direction and moves negatively downwards or to the left. This is seen in the motion of projectiles or objects in freefall.