To one dimension

Step 6

After completing the two-dimensional sequence, the one-dimensional case might be introduced as useful in some special cases. With the full understanding of the relationships between acceleration, velocity, displacement developed in the two-dimensional work, and the consequent ability to reason with arrows, the greater reliance on diagrams with which to reason, there should be less reason to trip ourselves in complex circumlocutions, or in what might appear to the children as somewhat ad-hoc sign conventions.