Lesson focus: speedHow fast something moves: the distance travelled each second, measured in metres per second (m/s). compares distanceHow far something has travelled, measured in metres (m). travelled with time taken; average speedThe total distance travelled divided by the total time taken. summarises a whole journey.
Calculate speed, distance or time using consistent units and distinguish average from instantaneous speedThe speed at one exact moment, like the reading on a car's speedometer..
Build on earlier learning
Balanced and unbalanced forcesYear 7 used resultants to explain changes in motion. This lesson measures motion quantitatively.
You will be successful when you can…
- State the speed equation with units.
- Rearrange for distance or time.
- Convert common units.
- Explain the limitation of average speed.
Words that carry the science
- speed
- Distance travelled per unit time.
- average speed
- Total distance divided by total elapsed time.
- instantaneous speed
- Speed at one particular moment.
- distance
- Total path length travelled.
- scalar
- A quantity with size but no direction.
Speed is a rate
01Speed is a rate
Speed = distance ÷ time. A speedHow fast something moves: the distance travelled each second, measured in metres per second (m/s). of 5 m/s means 5 metres of path are covered each second.
Metres with seconds give m/s; kilometres with hours give km/h. Convert before substituting when the requested unit differs.
02The relationship can be rearranged
distanceHow far something has travelled, measured in metres (m). = speed × time, while time = distance ÷ speed. Write the equation, substitute values with units, then calculate.
A value without a unit is incomplete because 12 m/s and 12 km/h describe very different speeds.
03Average speed compresses a journey
For a complete journey, average speed = total distance ÷ total elapsed time. Include a stop when it belongs to the stated journey.
A speedometer estimates instantaneous speedThe speed at one exact moment, like the reading on a car's speedometer.. The same average can hide different stops, accelerations and peak speeds.
Speed: distance and time
Speed tells you how far something travels each second. Speed = distance ÷ time. On a distance–time graph, a steeper line means a faster speed. The animation runs six times faster than real time.
Controls
Typical speeds: walking about 1.5 m/s, running about 5 m/s, cycling about 6–8 m/s.
Average speed with a stop
A cyclist covers 600 m in 120 s, waits 30 s, then covers 300 m in 50 s.
- Total distance = 600 + 300 = 900 m.
- Total time = 120 + 30 + 50 = 200 s.
- Average speed = total distance ÷ total time.
- 900 ÷ 200 = 4.5 m/s.
How to make the reasoning visible: The average is 4.5 m/s; the cyclist did not necessarily move at this speed throughout.
Calculate and interpret speed
Decide which quantity is unknown before rearranging.
| Journey | Distance | Time | Required |
|---|---|---|---|
| Runner | 400 m | 80 s | speed |
| Robot | ? | 12 s at 0.8 m/s | distance |
| Train | 36 km | ? at 72 km/h | time |
| Walk | 900 m | 15 min | speed in m/s |
Calculate each missing value.
Which value describes a whole interval?
Check your understanding
Answer from memory, then check against the suggested answer.
State the speed equation.
What is the SI unit?
How many seconds are in 3 minutes?
A car travels 30 km at 60 km/h then 30 km at 30 km/h. Is its average 45 km/h?
Why can average speed be non-zero after returning to the start?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A bus travels 12 km in 15 min, stops for 5 min, then travels 8 km in 10 min.
Calculate the average speed of the bus for the whole journey, in km/h. Show your working.
(4)Total for this question: 4 marks
Command word: For calculateA command word: work out a number answer, showing your working and the unit., show the equation, unit conversion and final unit.