Physics KS3/KS4: Average speed

In this video Professor Brian Cox introduces the concept of average speed and demonstrates how a distance-time graph can be used to represent a journey and calculate the average speed taken to complete it, using the example of a journey from home to school.

A simple three part distance-time graph is used to show the different phases of a journey and how the speed varies between each phase. The graph is then analysed to estimate the speed at different stages and then the average speed for the whole journey is calculated.

The video demonstrates how a graph can tell a story and give a more complete picture of the journey than just the average speed calculation.

Teacher Notes

Points for discussion:
This video is a good example of the importance of a graph in telling a story.

Students often do not see the point of a graph, but this video demonstrates how a graph can give us more information than just the calculation of average speed.

Suggested activities:
This video could be used once students are familiar with the speed equation and understand what it shows and how to use it.

Alternatively, this video could follow on from clip 3 in this series which introduces speed and how to calculate it.

Following on from this, students could practise interpreting distance-time graphs, starting with talk activities to describe the story behind each graph before calculating average speeds.

The level of difficulty of graphs could be increased. For example, by using more complex axes, using decimals or using points that do not fit neatly onto gridlines.

Students could be asked to calculate the speed at various stages of the journey shown in the graph. The practice of describing and explaining graphs would be beneficial for both KS3 and KS4 students.

Students could then construct their own graph of, for example, a journey of their own and could plot their own measurements of distances and times.

At KS4, this video could be used to revise and reinforce learning about distance-time graphs and how to calculate speed from them before moving on to study velocity-time graphs.

Curriculum Notes

Suitable for KS3, Combined Science and Physics GCSE in England, Wales and Northern Ireland and at National 4 and 5 in Scotland.

More from this series:

Newton’s First Law
Hooke’s Law
An introduction to the speed equation
The importance of checking mathematical answers
Relative speed