Consider a coordinate system in which the positive 𝑥-axis is directed vertically upward. What is the displacement of a particle 8.5 metres directly above the origin? What is the displacement of a particle 1.3 metres directly below the origin?
To start, let’s draw this positive 𝑥-axis that’s vertically oriented. On this axis that is vertically oriented, positive values are up and negative values are down. And the point that divides positive from negative is the origin.
We’re told that our first point is at a position 8.5 metres above the origin. Locating that on the axis, that would be here. The displacement of that point is equal to positive 8.5 metres. We recognize that by seeing the point oriented in its place on the axis.
We’re then told about the second point that is 1.3 metres below the origin. So starting at the origin, we count down 1.3 metres and see that point on the axis.
So the displacement of a point 1.3 metres below the origin is equal to negative 1.3 metres. For both these values, the displacement of the points is determined by its relationship to the origin.