The simplest of our graphs of motion are the distance-time (d-t) graphs:
From these graphs, we are expected to do the following:
- Describe the motion (similar to what is shown on the picture (above)
- Calculate the speed (in different sections)
Descriptions of Motion
STATIONARY
This is where the line of the graph is parallel to the x-axis (is horizontal):
The speed = 0 (zero), because there is no change in distance (Δd)
CONSTANT SPEED
This is where the line of the graph is a straight line on an angle. If the line is going upwards, the object is moving forwards. If the line is going downwards, the object is going backwards.
We can also use these to compare two speeds very quickly:
Runner A is going faster, because the gradient (slope) of the the graph is steeper for this runner. That meas they have a higher speed, which can be calculated:
These sections of a d-t graph can lead to a question where we need to calculate the speed, using the equation we met earlier in this topic:
RUNNER A
Δd = 600 metres
t = 100 seconds
v = 6 metres per second
RUNNER B
Δd = 600 metres
t = 150 seconds
v = 4 metres per second
ACCELERATING
If the line is curved, the speed is constantly changing - it is accelerating:
We cannot use the d-t- graph to do any calculations on this type of graph (until we learn more equations in Year 12 Physics)
