Describing motion
‘Describing motion’ is all about how you describe the movement of an object in motion.
Motion can be described in three ways:
 Speed
 Velocity
 Acceleration
Speed
The distance travelled by an object in a given time is called as the speed of that object.
Speed (or average speed) can be found out using the below equation:
Here,
 Average speed is measured in meters per second (m/s)
 Distance moved is measured in meters (m)
 Time taken is measured in seconds (s)
Here is a worked example:
The equation for speed can be rearranged to calculate other quantities as well:
Distance time graphs
 A distancetime graph shows how far something travels over a period of time.
 The vertical axis of a distancetime graph is the distance travelled from the start.
 The horizontal axis is the time from the start.
See the example below:
In a distancetime graph,
Speed = gradient of distance against time graph.
Hence, in the example above, the speed is 2.5m/s.
Speedtime graphs
A speedtime graph demonstrates the speed at which an object is travelling at a given time.
Exam questions are often asked on calculating the average speed in a speed time graph. This should be solved by:
 First finding the total distance travelled by calculating the area under the speed time graph
 And then dividing the total distance travelled by total time taken to travel it.
Acceleration
Acceleration is the rate at which an object changes speed.
Acceleration in a speedtime graph can be measured by calculating the gradient of the graph.
Another way of calculating the acceleration of the object isĀ using the formula:
Acceleration= change in speed/time taken
The SI unit for acceleration is m/s^{2 }(meters per second square)

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