Speed

Speed measures the distance travelled over a given period of time.

v = d / t

• d: the distance travelled
• t: the time taken
• v: the average speed

An example

A train travels 120 km in 2 hours:

v = 120 / 2 = 60 km/h

The unit is evident from the formula: kilometres divided by hours gives kilometres per hour. The unit is calculated as the number — this is a free and very effective way to check your work.

Average speed, instantaneous speed

The train did not travel at 60 km/h the whole way: it started, accelerated, and slowed down. 60 km/h is its average speed.

The instantaneous speed is the speed shown on the speedometer at a specific moment. This is the speed measured by a speed camera.

The two can be very different. On a journey with stops, the average speed is always lower than what the speedometer displays.

Converting km/h to m/s

This is the most useful conversion in all of mechanics, because the SI system uses m/s.

1 km/h = 1000 m / 3600 s = 1/3,6 m/s

So:

km/h  ->  m/s   :  diviser par 3,6
m/s   ->  km/h  :  multiplier par 3,6

A few reference points to help check a result:

km/h	m/s
3.6	1	slow walk
36	10	sprinter
90	25	road
1080	300	airliner

If you convert 90 km/h and get 324 m/s, you’ve multiplied instead of dividing. The reference point ‘motorway = 25 m/s’ is enough to realise this.