In this lesson, you take data by clicking on a person moving across the field of view in different directions and either walking forwards or backwards. Each of these will be plotted on a separate graph. The graphs show horizontal (across) distance on the y-axis plotted against time on the x-axis. You will also have an "All" graph showing all of your data. You probably use the words speed and velocity interchangeably as do most people. Scientists, however, are more particular. To them, speed is a number with units of distance divided by time. Velocity is speed plus direction.