Speed · Grade 9
Speed tells you how far something travels each second. Most journeys speed up and slow down, so we often work out an average speed. Distance–time graphs show a journey at a glance: the steeper the line, the faster the motion.
Key facts
- Speed = distance ÷ time (v = d/t). Rearrange to d = vt and t = d/v.
- Average speed = total distance ÷ total time (include any stops).
- To change km/h to m/s divide by 3.6; m/s to km/h multiply by 3.6.
- Distance–time graph: gradient = speed; a horizontal line means stationary; a curve means changing speed.
- Typical speeds: walking 1.5 m/s, cycling 6 m/s, car on a motorway 30 m/s, sound in air 340 m/s.
- Stopping distance = thinking distance (speed × reaction time) + braking distance.