GCSE-ready- Home
- Lessons
- KS3 Science
- Speed and distance-time graphs
Speed and distance-time graphs
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
How fast is a sprinter, a snail or a spacecraft? Speed tells us how far something moves each second.
Calculating speed
speed = distance ÷ time
Speed is often in metres per second (m/s) or kilometres per hour (km/h).
This gives the average speed: during a journey the actual speed may change.
Rearranged: distance = speed × time, and time = distance ÷ speed.
Speed is often in metres per second (m/s) or kilometres per hour (km/h).
This gives the average speed: during a journey the actual speed may change.
Rearranged: distance = speed × time, and time = distance ÷ speed.
Distance-time graphs
A distance-time graph shows a journey.
A straight sloping line: moving at a steady speed.
A steeper line: moving faster.
A flat (horizontal) line: stopped (stationary).
Speed = the gradient (slope) = distance up ÷ time across.
A straight sloping line: moving at a steady speed.
A steeper line: moving faster.
A flat (horizontal) line: stopped (stationary).
Speed = the gradient (slope) = distance up ÷ time across.
Relative motion
Motion depends on what you compare it with.
Two cars travel the same way at 30 m/s and 25 m/s. Compared with the slower car, the faster car moves away at only 30 − 25 = 5 m/s.
If they travel towards each other, they close the gap at 30 + 25 = 55 m/s.
Two cars travel the same way at 30 m/s and 25 m/s. Compared with the slower car, the faster car moves away at only 30 − 25 = 5 m/s.
If they travel towards each other, they close the gap at 30 + 25 = 55 m/s.
A runner covers 100 m in 20 s. Find her average speed.
- speed = distance ÷ time
- speed = 100 ÷ 20
Answer: 5 m/s
Speed = distance ÷ time. On a distance-time graph, the gradient is the speed: steeper is faster, flat means stopped. Relative speed: subtract speeds in the same direction, add them in opposite directions.
Check you have got it
Answer 7 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.