# 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:

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

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## 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.

## 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.
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## 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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