2.1: Uniform Linear Motion

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Linear motion means that an object moves along a straight line. "Uniform" simply means that the acceleration is 0 throughout the motion. In other words, the velocity remains constant/uniform. As a graph,

By definition, speed is the rate of distance being covered. In this simple case,

speed=distance/time

Therefore, by analogy,

velocity=displacement/time

The term displacement is simply the vector analog of direction. However, it is important to note that displacement only gives the distance from the initial and final position, not the length of the whole path as distance would. This will become clear when we discuss vectors in depth later. For illustration, we can imagine you walking 1 meter forward and then 1 meter backward. Though the distance you covered would be 2m, because your initial and final position are the same, your displacement is 0. From here, displacement will be denoted by \(\overrightarrow{s}\) and velocity by \(\overrightarrow{v}\). By convention, a little arrow is put on the top of a vector value, or the character is bold (e.g. v).

So,

\(\overrightarrow{s}=\overrightarrow{v}\cdot t\nonumber\) ∵ By rearranging

In the graph, this corresponds to the area between the x-axis and the line denoting the velocity. So if we take the area covered between t=0 to t=5, we get 20 meters. This might seem strange initially, but think about it for a moment. When this idea clicks, it would be easy to understand the generalization of this equation.