In motion processes, the position of bodies changes relative to a reference system. If the distance travelled by the body is straight and its velocity does not change, the movement is said to be rectilinear and uniform. The movement of a bubble in an inclined Mikola tube is also of this type if the angle of inclination does not change.

A uniform rectilinear motion is achieved when the forces acting on the moving body are eliminated or balanced. The sum of the forces acting on the body is zero: ∑F=0N. The SI unit of the resulting speed is the ratio of metres to seconds (m/s), but kilometres per hour (km/h) is more commonly used in everyday life. The conversion between the two units is as follows:

Since speed is the distance travelled in one second, distance is in direct proportion to time at a constant speed (s=v∙t). If you plot the distance as a function of time, you get a line whose gradient corresponds to the speed of the body. A higher speed means that the distance travelled within the same time period is greater, so the slope of the line in the distance-time graph is also greater. The graph in the illustration shows the relation between speed and time of three objects: v1>v2>v3.

If we also draw the velocity-time graph, the result is a horizontal line, since there is no force acting on the body that could cause acceleration or deceleration. The product of the constant velocity and the elapsed time gives the area of the rectangle under the "curve" that corresponds exactly to the distance travelled by the body. It is worth noting that the area under the curve of the velocity-time graph can be used to provide the value of the distance travelled by the body for any type of movement besides the uniform motion.