14. Dublin’s port tunnel has a length of 4·5 km and runs from Dublin port to the M50 motorway. The speed limit inside the tunnel is 80 km/h.
A traffi c management system takes a photo of a vehicle when it enters the tunnel and when it leaves the tunnel. The time a driver takes to make it between the two points is used to determine if they have been speeding.
The table below shows the entry and exit times for three vehicles. Did any of these vehicles break the speed limit?
Vehicle Time of entry Time of exit A 08:22 B 13:48 C 18:05
08:26 13:51 18:08
14.3 Rate of change graphs
By the end of this section you should: ● be able to draw distance/time graphs and tables
● be able to use the distance/time graphs and tables to calculate speed
● understand that the relationship between distance and time is proportional for a constant speed
Distance/time graphs Discuss and discover
Work with the person beside you to consider the table below. It shows the distance of an object from a start point over a period of time. Use it to answer the following questions:
Time (s) Distance from start (m)
0 1 2 3 4 5 6 7 8 0 10 20 30 40 50 60 70 80
(i) Do you notice a pattern in the table? (ii) What happens for every 1 second increase in time? (iii) Is this constant across the 8 seconds? (iv) How could you describe the speed of the object?
We were introduced to linear patterns, rate of change and slope in Section A, Unit 7.
(v) Use the speed formula to calculate the speed of the object at 2, 4 and 6 seconds. What do you notice? What does this tell you about the speed of the object?
(vi) Draw a graph to represent the distance travelled by the object against time. What does the graph look like? What is the rate of change of the graph?
(vii) What is the slope of the graph? What do you notice about the rate of change, the slope of the graph and the speed of the object?
Having completed the activity above, we can see that when the rate of change remains the same the graph will always be a straight line. This type of pattern is called a linear pattern.
A linear pattern is one in which the rate of change remains constant. Section B Moving forward 221 ● understand that speed is a rate of change
● understand the steeper the line the greater the rate of change
● be able to tell the story of a distance/time graph ● understand and tell the story of other rate of change graphs