If an object moves along a straight line, the distance travelled can be represented by a distance-time graph.
Calculate the speed of the object represented by the green line in the graph, from 0 to 4 s.
change in distance = (8 - 0) = 8 m
change in time = (4 - 0) = 4 s
Calculate the speed of the object represented by the purple line in the graph.
change in distance = (10 - 0) = 10 m
change in time = (2 - 0) = 2 s
The table shows what each section of the graph represents:
|Section of graph||Gradient||Speed|
|D||Zero||Stationary (at rest)|
If an object is accelerating or decelerating, its speed can be calculated at any particular time by:
As the diagram shows, after drawing the tangent, work out the change in distance (A) and the change in time (B).
Note that an object moving at a constant speed is usually changing direction continually, eg moving in a circle. Since velocity has an associated direction, these objects are also continually changing velocity, and so are accelerating.
The Moon and other satellites orbit the Earth at a constant speed, but are accelerating towards the Earth.