Average Speed

Speed in Everyday Life

Speed tells us how fast something is moving — how much distance it covers in a given time.

A car travelling at 60 km/h covers 60 kilometres in one hour.

The Formula

v = \frac{d}{t} \qquad d = v \times t \qquad t = \frac{d}{v}

Measuring Speed

1
Measure the distance
Use a ruler, measuring tape, or known track length.
2
Measure the time
Use a stopwatch. Start when the object starts moving, stop when it finishes.
3
Calculate
Divide distance by time.

A 100 m sprinter finishes in 12.5 s

$v = \dfrac{100}{12.5} = 8 \text{ m/s}$

Converting to km/h: $8 \times 3.6 = 28.8 \text{ km/h}$

Why "Average" Speed?

In real journeys, speed changes constantly. Average speed uses total distance and total time.

A bus trip: 20 km in 30 min, then stop for 10 min, then 40 km in 50 min

Total distance = 60 km

Total time = 90 min = 1.5 h

Average speed $= 60 \div 1.5 = 40 \text{ km/h}$

Note: the 10-min stop is included in total time.

Real world: speed cameras and road safety

A speed camera on a motorway measures that a car travels 50 m between two sensors in 1.8 s. Average speed $= \frac{50}{1.8} \approx 27.8\text{ m/s} = 100\text{ km/h}$ . The 100 km/h speed limit means this car is exactly at the limit. Average-speed cameras over longer distances (e.g. 5 km sections) are even harder to evade by briefly slowing down.

Real world: planning a road trip

A family drives 450 km on a motorway at an average speed of 100 km/h. Journey time $= \frac{450}{100} = 4.5$ hours. They also add 30 minutes for stops, giving a realistic total of 5 hours. Drivers use speed-distance-time every time they plan a trip or estimate arrival.

Real world: athletics — judging pace

A marathon runner aims to complete the 42.195 km race in 3 hours. Required average pace $= \frac{42.195}{3} \approx 14.1\text{ km/h} = 3.9\text{ m/s}$ . Coaches use $v = d/t$ to set per-kilometre target times and to adjust strategy during the race.

Tip For average speed, always use total distance and total time — including any stops. The average speed is never the average of two different speeds unless equal distances were covered at each speed.

Practice Problems

A ball rolls 15 m in 3 s. What is its average speed?

Show answer

v = 15 ÷ 3 = 5 m/s.

A plane flies at 800 km/h. How far does it travel in 45 minutes?

Show answer

t = 0.75 h. d = 800 × 0.75 = 600 km.

A swimmer covers 200 m at 2 m/s, then 100 m at 1.5 m/s. What is the average speed for the whole swim?

Show answer

t1 = 100 s, t2 = 66.7 s. Total: 300 m in 166.7 s → v ≈ 1.8 m/s.