M4.5.6 For the adult
Speed, distance and time
A car drives 60 km at 60 km per hour, then 40 km at 20 km per hour. What is its average speed?
What this chapter teaches
Speed is distance divided by time, in a compound unit such as km/h. The time must be in the unit of the speed. Divide km/h by 3.6 to get m/s. For a journey in two parts, the average speed is the total distance divided by the total time, not the mean of the two speeds.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why must you change 90 minutes into hours before finding a speed in km/h?
- Why is the average speed of a journey at 60 km/h and 20 km/h not 40 km/h?
- How do you change a speed in m/s into km/h, and why does that number work?
After the chapter, the learner can
- Use speed = distance ÷ time with the right units
- Change km per hour into metres per second
- Find the average speed for a journey in two parts
What they copy into their notebook
Speed = distance ÷ time. Distance = speed × time. Time = distance ÷ speed. km/h to m/s: ÷ 3.6. Average speed = total distance ÷ total time.