When an object does not move at a constant speed, using a straight line to represent its motion on a distance–time graph is not realistic. A curved line more accurately suggests the type of motion that is taking place.
Look at Fig. 17.2.3. (a)
(b)
17.2
Fig. 17.2.3 (a) A person dropping an apple from a building
Time (s)
z The distance the apple falls every second changes as the journey progresses (a).
z The curve of the distance–time graph for this motion shows us that for every second of the motion, the object is covering a larger distance (b).
This graph shows that the apple’s speed is getting greater with time. In other words, the apple is accelerating.
Visualising acceleration
The car shown in Fig. 17.2.1 has an acceleration of 2.5 m/s2, meaning that for every second of its journey its speed is increasing by 2.5 m/s. Using this information, complete the table below for the car’s distance travelled and its speed at every second of its journey, starting at a speed of 15 m/s:
Time 0
1
2 3 4
Use these values to draw two graphs: 1. The speed of the car against time.
2. The distance of the car against time. In both graphs, put time on the x-axis. What do the shapes of these graphs tell you about the motion of the car?
