Consider the problem of motion in a straight line with constant acceleration. A complete description of the motion involves 5 quantities: the initial velocity ($u$), the time elapsed ($t$), the displacement ($s$), the final velocity ($v$), and the acceleration ($a$). These quantities are, of course, not independent of one another - they are related via physical equations and knowing some can let us know others. So, an important question is: how many of these quantities do we need to know in order to know everything about the motion? It turns out, we need to know 3 of the 5 quantities. For instance, if we know the initial velocity, the acceleration, and the time elapsed, then we can find the both the final velocity and the displacement. We do so by using the following two equations of motion: (i) $v-u=at$ (ii) $s=ut+{1/2}at^2$ Both the above equations arise from how we define acceleration and velocity. Acceleration, being the rate of change of...