1Speed changes on a journey
On a real journey your speed keeps changing: you speed up, slow down and stop at junctions. A car's speedometer shows the speed at each moment.
speedometer
Lesson 18.2 notes
Average speed on a journey
Average and instantaneous speed, speed cameras, and changing units.
2Average speed
Average speed = total distance ÷ total time. A 1200 m walk to school that takes 15 minutes (900 s) has an average speed of 1200 ÷ 900 ≈ 1.3 m/s.
average speed
3Instantaneous speed
The speed at one moment is the instantaneous speed. A car may reach 13 m/s between junctions but average far less, because the stops count too.
instantaneous speed
4Speed cameras
A fixed speed camera checks a car's speed at one place. Average speed cameras time a car between two cameras a known distance apart: average speed = distance ÷ time.
average speed camera
5Units of speed
Speeds can be in m/s, km/h or mph. To change m/s into km/h, multiply by 3.6, so 10 m/s = 36 km/h. A 30 mph limit is about 13 m/s.
km/hmph
6Matching units
Use matching units: metres with seconds give m/s, and kilometres with hours give km/h. Change minutes into seconds first by multiplying by 60.
7Planning a journey
Rearrange to plan: time = distance ÷ average speed. A 30 km trip at an average of 60 km/h takes 30 ÷ 60 = 0.5 hours: half an hour.
Explore: Journey lab
Choose a journey and drive it. Watch the speedometer change while the average speed is worked out as you go.
Common mistake: Average speed isn't the average of two speeds
Drive 60 km at 60 km/h (1 hour), then 60 km at 30 km/h (2 hours). The average speed isn't 45 km/h: it's the total distance ÷ total time = 120 km ÷ 3 h = 40 km/h.
Remember
- average speed = total distance ÷ total time, including any stops.
- Instantaneous speed is the speed at one moment, shown on a speedometer.
- Keep units matched: metres and seconds give m/s; kilometres and hours give km/h. To change m/s into km/h, multiply by 3.6.
Going further at GCSE
GCSE: Distance, speed and velocity → GCSE: Distance–time graphs →