The x-axis on a position-time graph shows the position of an object over time. The reason why it only shows positive values is because position is typically considered in terms of relative distance from a reference point, and this reference point is usually labeled as 0 on the graph. In other words, when an object is to the left of the reference point, its position is represented by a negative value on the x-axis, and when it is to the right, its position is represented by a positive value. However, in the case of a position-time graph, the reference point is often chosen to be the starting point of the object's motion, which is always a positive value as the object cannot occupy negative space. Therefore, the x-axis only shows positive values to represent the object's position as it moves away from the starting point.
