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 burgundy-line in the graph, from 0 to 2 s.
change in distance = (10 − 0) = 10 m
change in time = (2 − 0) = 2 s
speed = 10 ÷ 2
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).
It should also be noted that an object moving at a constant speed but changing direction continually is also accelerating. Velocity, a vector quantity, changes if either the magnitude or the direction changes. This is important when dealing with circular motion.