Units of speed
Metre per second and kilometre per hour.
Two speeds
A runner's speed is told in metres and seconds. A bus's speed is told in kilometres and hours.
Why do the units differ?
What this lesson covers
The idea
Since speed is distance/time, the SI unit of speed is metre/second (m/s); with distance in kilometres and time in hours, speed is in kilometre/hour (km/h).
Two speeds
A runner's speed is told in metres and seconds. A bus's speed is told in kilometres and hours.
Why do the units differ?
- Speed is distance divided by time, so its unit has a distance unit and a time unit
- Because runners and buses are different colours
- Units are chosen by luck
Build the unit
Pick a distance unit and a time unit. Build m/s and then km/h.
Distance unit over time unit
Metres over seconds made m/s. Kilometres over hours made km/h.
Since speed is distance/time, the SI unit of speed is metre/second (m/s); with distance in kilometres and time in hours, speed is in kilometre/hour (km/h).
Notes
Since speed is distance/time, the SI unit of speed is metre/second (m/s); with distance in kilometres and time in hours, speed is in kilometre/hour (km/h).
Check yourself
What is the SI unit of speed?
Distance in kilometres and time in hours give the unit of speed …
Which unit is most suitable for the speed of a bus on a highway?
The speedometer of a car shows speed in …
- km/h. km/h is also used, but it is not the SI unit.
- m/s — correct. Yes!
- m/h. This is not the SI unit of speed. The SI unit is metre/second.
- m/s. m/s is the unit when distance is in metres and time in seconds.
- s/m. That is upside down. Speed is distance divided by time.
- km/h — correct. Yes!
- km/h — correct. Yes!
- mm/s. That is a tiny unit, used for very slow things.
- cm/min. That is a very small unit, not used for a fast vehicle.
- m/s only. A car's speedometer shows km/h.
- km/h — correct. Yes!
- kg/h. kg is a unit of mass, not of distance.