Physics KS3/KS4: Relative speed

In this video Professor Brian Cox introduces the concept of relative speed using the context of vehicles travelling on the motorway.

Visual prompts and reference to students’ likely existing knowledge help to explain how to calculate relative speed when vehicles are travelling in the same direction and when travelling in opposite directions.

The video reminds students of the need to sense-check the answers calculated and assess whether or not they sound reasonable.

The video highlights the need to talk about speed using specific language like ‘relative’, and highlights the concept that we are constantly moving, even when at rest, with the movement of the Earth.

Teacher Notes

Points for discussion:
Relative motion (speed) is covered at KS3 but not explicitly at KS4.

Suggested activities:
The importance of relative speed could be investigated with reference to collisions. Students could calculate the relative speed in different collisions and suggest how this would impact on a collision.

Further observations of relative speed could be discussed, for example a train passing a car in the opposite direction.

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
Average speed