Who is faster?

Watch a 100 metre race on a straight track. All the runners start together, but soon they are spread out. How do you decide who is fastest?

Whoever is ahead at a given moment is running faster. In other words, the one who has covered more distance in the same time is faster. We say the faster runner has a higher speed.

But what if two races are over different distances, or take different times? Then we compare the distance covered in one unit of time: one second, one minute or one hour.

The distance an object covers in unit time is its speed. If we know the total distance and the total time:

• Speed = Total distance covered ÷ Total time taken

Distance is measured in metres (SI unit) and time in seconds, so the SI unit of speed is metre per second, written m/s. If distance is in kilometres and time in hours, the unit is kilometre per hour, written km/h — the unit you see on road signs.

The book's example: Swati's school is 3.6 km from her house, and she cycles there in 15 min. Change both into metres and seconds first: 3.6 km = 3600 m, and 15 min = 900 s. Speed = 3600 m ÷ 900 s = 4 m/s. Always make the units match before you divide.