‘Describing motion’ is all about how you describe the movement of an object in motion.
Motion can be described in three ways:
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:
- 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 distance-time graph shows how far something travels over a period of time.
- The vertical axis of a distance-time graph is the distance travelled from the start.
- The horizontal axis is the time from the start.
See the example below:
In a distance-time graph,
Speed = gradient of distance against time graph.
Hence, in the example above, the speed is 2.5m/s.
A speed-time 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 is the rate at which an object changes speed.
Acceleration in a speed-time graph can be measured by calculating the gradient of the graph.
Another way of calculating the acceleration of the object is using the formula:
Acceleration= change in speed/time taken
The SI unit for acceleration is m/s2 (meters per second square)
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