Speed and Velocity

Velocity is a vector quantity so it has direction. Speed is just a scalar.

Consider this example: A man walks from his house to a shop.

Notice that distance traveled is a scalar whilst displacement is a vector.

Of course what we have calculated above is the average speed and the average velocity for the journey. Both of these quantities may vary from instant to instant.

Consider this example: A man and a dog have a race over 60m. Here is a velocity time graph for the race.

• The speed at any instant is the "rate of change of displacement" at that instant.

• It is how quickly the displacement is changing at an instant.

• It is the gradient of a distance time graph at a particular time.

• If s is displacement then v = ds/dt