Uniform and non-uniform motion. Speed and its units
If a body covers equal distances in equal intervals of time, the motion is uniform; otherwise it is non-uniform. In uniform motion speed is the ratio of the distance covered to the time taken: v = s : t. From this s = v · t and t = s : v. The SI unit of speed is m/s; km/h is also widely used. They are related by 1 m/s = 3.6 km/h: to go from km/h to m/s divide by 3.6, to go from m/s to km/h multiply by 3.6. For example 72 km/h = 72 : 3.6 = 20 m/s. Real motion is almost always non-uniform, so we use average speed: the whole distance divided by the whole time (not a plain average of the speeds!). Speed is shown by a speedometer.
“Speed with a stopwatch”: a pupil walks 10 m at a normal pace while partners time it with a stopwatch; the speed is v = 10 : t, then converted to km/h. Safety: do not run; do it in an obstacle-free place (hall or yard).