Speed and velocity

Distance is how far an object moves. It does not include an associated direction, so distance is a scalar quantity.

Speed is the rate of change of distance – it is the distance travelled per unit time. Like distance, speed does not have an associated direction, so it is a scalar quantity.

Typical speeds - When people walk, run, or travel in a car their speed will change. They may speed up, slow down or pause for traffic. 

It is not only moving objects whose speed can vary. The speed of the wind and the speed of sound also vary. A typical value for the speed of sound in air is about 330 m/s.

To work out an object’s speed you need to know the distance it has travelled, and the time taken.  

Calculate speed using the equation: Speed = distance/time.

Average speed is calculated as the distance traveled over the total time of travel.

For example:

If you by car travelled from Paje to Potoa, as distance of 72.5 km, in 1 hour, your average speed would be: 

Average speed = 72.5/1 = 72.5 km/hr. 

That's not to say that at every point of the journey, you will be travelling at 72.5 km/hr. Some of the time you will be travelling faster and at other times slower.  

The velocity of an object is its speed in a particular direction.  Velocity is a vector quantity because it has both a magnitude and an associated direction. To calculate velocity, displacement is used in calculations, rather than distance.

Unlike distance, which is a scalar quantity, displacement is a vector quantity. 

It includes:

• the distance travelled, measured in a straight line from start to finish
• the direction of the straight line

Images: Harrage, E. (2023). Speed and velocity (CC BY)

Example:

During a 4 s time interval, the runner’s position changes from x₁ = 60 m to x₂= 40 m. What is the runner’s average velocity?

Solution:

The change in the displacement of the runner is calculated as follows:

Δx = x₂ – x₁ = 40 m – 60 m = -20 cm

Δt = 4 s

v_average = Δx/Δt = -20 m/ 4 s = -5 m/s