Uniform Motion along a Straight Line

This represents an important situation. In this case, the acceleration vector is constant and lies along the line of the displacement vector, so that the directions of v and a can be specified with plus and minus signs. If we represent the displacement by s (positive) if in the positive direction, and negative if in the negative direction), then the motion can be described with the five equations for the uniformly accelerated motion:

Direction is important, and a positive direction must be chosen when analyzing motion along a line. Either direction may be chosen as positive. If a displacement, velocity, or acceleration is in the opposite direction, it must be taken as negative.