Units of speed

A road sign says 90 km in each hour. A cyclist covers 12 metres every second. Which is faster?

The idea

90 km in each hour is faster than 12 m each second. To compare, write both in one unit.

Scientists measure speed in metres per second, written m/s. Per means in each. Speed can also be measured in kilometres per hour, km/h.

1 km = 1000 m. 1 h = 3600 s. So 1 m/s = 3600 ÷ 1000 = 3.6 km/h. In 1 h, 1 m/s covers 3600 m. That is 3.6 km.

Multiply m/s by 3.6 to get km/h. Divide km/h by 3.6 to get m/s.

90 km/h = 90 ÷ 3.6 = 25 m/s. 25 is more than 12. So 90 km/h is faster than 12 m/s.

Try the lab to finish this lesson.