What is speed?
We often hear about accidents caused by vehicles travelling too fast. It is exactly why speed limits are set on main roads.
So what, precisely, is "speed"?
Speed
Speed is the rate of covering distance.
speed = distance ÷ time
That is, speed is the distance an object moves in unit time.
The unit
Distance is given in metres (m) and time in seconds (s), so the unit of speed is metres per second. The short way of writing it is m s−1.
How to read the unit
m s−1 is another way of writing m ÷ s. So a speed of 5 m s−1 means "5 metres every second".
Speed is not always the same
In road traffic, a vehicle usually cannot hold one steady speed. When there are many other vehicles the driver has to slow down, and at pedestrian crossings the vehicle has to stop altogether.
What a car's speedometer shows is the speed at that moment. Over the next two pages we look separately at motion where the speed does stay the same, and motion where it does not.