Example questions
If you have got to grips with speed-time graphs you should now be ready to tackle a typical examination question. The question below is designed to test how good you are at interpreting a speed-time graph. It also tests your knowledge of what is meant by speed and acceleration.
The speed-time graph in the question shows how the speed of a bus changes during part of a journey. See how you get on with the question. Then scroll down to see if you managed to get the correct answers.
Question 1
 |
bus graph |
Look at each of the lettered sections OA, AB, BC, CD, DE and EF on the speed-time graph.
(a) Choose the correct word from the table below to describe the motion taking place in each section.
| acceleration |
steady speed |
stationary |
(i) OA (ii) AB (iii) BC (iv) CD (v) DE (vi) EF
(b) At what time during the journey did the bus reach its greatest speed?
(c) How long was the bus stopped for during its journey?
(d) During which section of the journey did the bus have the greatest acceleration?
(e) Calculate the acceleration of the bus during section DE.
(f) Which section could be described as a deceleration?
The Answer
(a)
| OA |
The speed of the bus is changing. Therefore, according to the definition, bus has an acceleration. |
| AB |
The speed does not change. The bus has a steady speed of 10 m/s. |
| BC |
The bus is slowing down. Its speed is changing. Therefore, according to the definition, the bus has an acceleration. |
| CD |
The speed of the bus is 0 m/s over this section. The bus is stationary. |
| DE |
The speed of the bus is changing. The bus has an acceleration. |
| FG |
The speed does not change. The bus has a steady speed of 5 m/s. |
(b) After 5 s
(c) 3 s
(d) BC. This part of the graph has the steepest slope.
(e)
| Acceleration in section DE |
= |
change in speed/time taken |
| Change in speed |
= |
5 m/s |
| Time taken |
= |
5 s |
| Acceleration |
= |
5/5 |
| |
= |
1 m/s2
|
(f) BC. The bus is slowing down. This type of acceleration is referred to as deceleration.
Examiner's Note
In the Credit Level examination you are expected to be able to use information from a speed-time graph in calculating the distance travelled by the object. The important fact that you have to remember is that the area under a speed-time graph represents the distance travelled.
Happening Hint
It is helpful to break the area under the speed-time graph into triangular and rectangles when you are calculating the distance travelled.
 |
speed/time graphs |
The area under graph A is represented by the area of a rectangle. The distance travelled is equal to ( 3 x 10) = 30 m.
The area under graph B is represented by the area of a triangle. The distance travelled is equal to ( 1/2 x 5 x 6 ) = 15 m.
The area under graph C is represented by the sum of the area of a triangle and a rectangle.
The distance travelled is equal (1/2 x 3 x 8) + (2 x 8 ) = 28 m.
Question 2 (Credit Level)
Try answering this question to see if you can work out the distance travelled from a speed-time graph. This is a Credit Level task. Once you have tried it, scroll down to the answer to see if you did the calculation correctly.
|
bus graph |
(a) What is the distance travelled by the bus during its 25 s journey?
(b) What is the average speed of the bus during its 25 s journey?
The Answer
(a) The speed-time graph can be divided up into triangular and rectangular areas as shown.
 |
bus graph |
| Area under section OA |
= |
1/2 x 5 x 10 |
= |
25 |
| Area under section AB |
= |
5 x 10 |
= |
50 |
| Area under section BC |
= |
1/2 x 3 x 10 |
= |
15 |
| Area under section CD |
= |
0 |
= |
|
| Area under section DE |
= |
1/2 x 5 x 5 |
= |
12.5 |
| Area under section EF |
= |
4 x 5 |
= |
20 |
| |
|
|
|
|
| Total area |
= |
122.5 |
|
|
| Distance travelled in 25 s |
= |
122.5 m |
|
|
(b)
| Average speed of bus |
= |
distance travelled / time taken |
| |
= |
122.5/25 |
| |
= |
4.9 m/s |
How did you get on? Do you need more practice at answering questions?
Now try a Test Bite
| 
