Step by step
The question
Question: the distance-time graph joins (0, 0) (3, 90) (4, 90), find the speed of each part.
distance-time graph: (0, 0) (3, 90) (4, 90)
1 Read the graph
Across: the time in hours.
Up: how far from the start, in km.
Going up: moving away from the start.
Flat: stopped.
Going down: coming back.
2 Speed of each part
Speed is the distance divided by the time.
On the graph that is the slope: up over across.
speed = distancetime = slope
Part A: 90 km in 3 h.
90 divided by 3 is 30 km/h.
A: speed = 903 = 30 km/h
Part B is flat.
The distance does not change: 0 km in 1 h.
The speed is 0: stopped.
B: speed = 01 = 0 km/h
Careful!
The flat part is high on the graph, but height is distance, not speed.
Flat means stopped.
Careful: not speed = 90 km/h ✗
3 Check
Let's check: speed times time gives back each distance: 90.
So it is right!
30 × 3 = 90
Let's look back.
First: read the graph.
Then: speed of each part.
Last: check.
Answer
Careful: not speed = 90 km/h ✗