Speed · Usually met around P6
Distance–time graphs
Flat is a rest; steeper is faster.
How to do it
Method
Flat is a rest; steeper is faster
4 km in 20 minutes: 4 × 3 = 12 km/h
Another way
In hours
4 ÷ 13 = 12
In full
A distance–time graph has time along the bottom and distance from the start up the side. First check what each square stands for. A line sloping up is moving on, a flat line is a rest (time passes, the distance stays the same), and a line sloping down is coming back. The steeper the line, the faster. For the speed of one part, read its distance and its time: speed = distance ÷ time; the time is read in minutes, so for km/h turn it into hours first: 4 km in 20 minutes, 20 minutes = 13 hour, 4 ÷ 13 = 12 km/h. Another way: 4 km in 20 minutes, and 60 minutes is 3 times as long, so 4 × 3 = 12 km in an hour.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
The graph shows Chi-keung's journey. How many minutes did Chi-keung rest on the way?
Show the answer and working
Answer: 20 minutes
The flat part B to C is the rest: from minute 20 to 40, 20 minutes.
Example 2
The graph shows Tsz-hin's journey. From A to B, what was Tsz-hin's speed in km/h?
Show the answer and working
Answer: 9 km/h
3 km in 20 minutes; an hour is 60 ÷ 20 times as long: 9 km/h.
Example 3
The graph shows Tsz-hin's journey. From C to D, Tsz-hin's speed was 5 km/h.
True or false?
Show the answer and working
Answer: False
C to D: 3 km in 20 minutes: 3 ÷ (20 ÷ 60) = 9 km/h.
Practise it in the game →Speed · Free to start, unlimited questions, no download