Average speed and velocity
A runner goes 80 m out and then 40 m back. The run takes 20 s. How fast was the run?
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The average speed was (80 + 40) ÷ 20 = 120 ÷ 20 = 6 m/s.
The rule
Average speed is total distance divided by time. Average velocity is displacement divided by time.
Speed has no direction. Velocity has one, shown by a plus or minus sign. Both use metres per second, m/s, or kilometres per hour, km/h.
To change units, use 1 m/s = 3.6 km/h. So 36 km/h is 36 ÷ 3.6 = 10 m/s.
In the run above, distance is 80 + 40 = 120 m. Displacement is 80 − 40 = +40 m.
| Quantity | Working | Answer |
|---|---|---|
| Average speed | 120 ÷ 20 | 6 m/s |
| Average velocity | +40 ÷ 20 | +2 m/s |
Take a straight line. Speed and velocity are equal in size only with no turning back.
Watch out!
Zero average velocity does not mean the object stood still. A swimmer swims up the pool and back. The swim ends where it began. The displacement is zero.