Speed and motion
Speed is the distance an object covers divided by the time it takes, and it tells you nothing about the direction the object is moving.
Practise this conceptSpeed tells you how quickly something covers ground: divide the distance travelled by the time it took, and that's it. A car covering 120 km in two hours has an average speed of 60 km/h, regardless of whether it drove that whole time at a steady pace or sped up and slowed down along the way.
That word average matters. A speedometer shows instantaneous speed — the speed right now — while distance divided by total time only ever gives you the average over the whole trip. The two can be very different if the motion isn't steady.
Speed is a scalar: it has a size but no direction. Two cars leaving the same point at 60 km/h, one heading north and one heading south, have identical speeds but are clearly not doing the same thing — that distinction is exactly what velocity, a vector, is for.
Key ideas
Requires: Physical quantities and units
Formulas
| Symbol | Name | Unit |
|---|---|---|
| speed | m/s (metre per second) | |
| distance | m (metre) | |
| time | s (second) |
Common misconceptions
- Speed is calculated by dividing time by distance, not distance by time.
- The steepness of a distance-time graph shows how far an object has travelled, not how fast it is going.
- Average speed can be found by simply averaging the starting and ending speeds, regardless of how long each part of the journey took.