Calculating Average Speed

Average speed is the total distance travelled divided by the total time taken. It is one of the most frequently tested calculations in IGCSE Physics.

What is the equation?

$v = \frac{s}{t}$

where $v$ is speed in m/s, $s$ is distance in m, and $t$ is time in s.

Average speed covers the entire journey, including any stops. If a car travels 150 km in 2 hours (with a 10-minute stop in between), the average speed uses the full 2 hours.

How do you convert km/h to m/s?

Divide by 3.6. One kilometre is 1000 m and one hour is 3600 s:

$72\ \text{km/h} = \frac{72 \times 1000}{3600} = 20\ \text{m/s}$

To convert m/s to km/h, multiply by 3.6.

Worked example

A cyclist rides 2400 m in 4 minutes. Calculate the average speed. [3]

Step 1. Convert time: $4 \times 60 = 240\ \text{s}$

Step 2. Substitute: $v = \frac{2400}{240}$

Step 3. $v = 10\ \text{m/s}$

Marking points: M1 for equation or correct substitution. M1 for converting minutes to seconds. A1 for 10 m/s with unit.

These marking points are for this original example and are not an official Cambridge mark scheme.

Common errors and how to correct them

• Forgetting to convert minutes to seconds. Using $t = 4$ instead of 240 gives 600 m/s, faster than the speed of sound. Always check whether the answer is physically reasonable.
• Confusing total distance with displacement. Average speed uses total distance (scalar), not displacement (vector). A journey of 5 km north then 5 km south covers 10 km total distance.
• Mixing units. If distance is in km and time in hours, the answer is in km/h. If the question asks for m/s, convert before substituting.

How to apply this in an exam

Write the equation, show the substitution (including any unit conversion), then state the answer with its unit. This three-line method secures the maximum number of available marks even if the arithmetic contains an error.