Speed

You run for the bus, a car overtakes you, and your bike rolls gently down the hill: everywhere around you things are moving, sometimes slowly, sometimes fast. Speed gives you a tool to say exactly how fast something is, and to compare motions with each other.

Distance per time: v = s/t

Speed tells you how much distance is covered in a certain time. The formula is v = s/t: speed is distance divided by time. If you run 100 m in 20 s, then v = 100 m / 20 s = 5 m/s. That means: every second you get 5 metres further. The unit m/s shows you the formula itself, metres divided by seconds.

Converting m/s and km/h

Road signs show km/h, but in physics you often work with m/s. Luckily the conversion is simple: from m/s to km/h you multiply by 3.6. Where that factor comes from is written in the units: an hour has 3600 seconds, a kilometre has 1000 metres, and 3600 divided by 1000 gives 3.6. So 10 m/s is exactly 36 km/h. The other way round you divide by 3.6: 72 km/h is 20 m/s.

Average or snapshot?

If you cycle 12 km in one hour, your average speed is 12 km/h. But you were not equally fast the whole time: at a red light your speedometer showed 0, downhill maybe 30 km/h. What the speedometer shows at a single moment is the instantaneous speed. For comparison, some everyday values: a pedestrian manages about 5 km/h, a bicycle around 15 to 20 km/h, a car in town 50 km/h.