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:

Speed 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:

Worked example

The equation for speed can be rearranged to calculate other quantities as well:

Speed distance time pyramid

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:

Distance time graph

In a distance-time graph,

Speed = gradient of distance against time graph.

Hence, in the example above, the speed is 2.5m/s.

 

Speed-time graphs

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:

  1. First finding the total distance travelled- by calculating the area under the speed time graph
  2. And then dividing the total distance travelled by total time taken to travel it.
Speed time graph

Acceleration

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)

Exam tip!

Exam questions are often asked on speed time graphs and distance time graphs. These include:

  • Calculate the average speed of the object
  • Calculate acceleration of the object in the first 2 seconds
  • Calculate deceleration of the object in the last 2 seconds etc.

 

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