The formula
v = d / t
| Symbol | Quantity | Unit |
|---|---|---|
| v | Speed | m/s |
| d | Distance | m |
| t | Time | s |
What it means
Average speed is the distance covered divided by the time taken. Average, not instantaneous: cover 100 km in two hours and your average is 50 km/h, even if you did 120 on one stretch and stopped for fuel on another.
How to work it out by hand
- Check the distance and time are in compatible units
- Divide the distance by the time
- The answer is in whatever units you used
What is worth knowing
Mixing units is the commonest mistake: kilometres with seconds, or metres with hours. Working in metres and seconds gives m/s; multiply by 3.6 for km/h. Watch too for the difference between distance travelled and displacement: go out and come back and your displacement is zero, so your average velocity is zero while your average speed certainly is not. Physics keeps the two words apart for exactly that reason.