To find how fast something moves you need two measurements: how far it goes and how long it takes. Distance is measured along the path the object actually travels. A ruler suits short distances such as a trolley on a ramp, a tape measure suits a classroom or playground, and a trundle wheel suits long distances such as a running track. Always measure from the start line to the finish line.
Time is measured with a stopwatch. A person starting and stopping a stopwatch adds an error of about 0.2 s because of reaction time. This is a large share of a short time, so a longer distance gives a smaller percentage error. Repeating the measurement and taking a mean also helps.
Light gates remove the human error and give more accurate results. A light gate starts and stops a timer automatically when a beam is broken. For a trolley, a card of known length is fixed on top. The gate records how long the card takes to pass, and the speed is the length of the card divided by that time. Speed cameras and video with a known frame rate are other ways of timing fast motion.
