A cheetah can hit 30 metres every second; a snail plods along at less than a millimetre. Both are moving — the difference is their speed, which simply tells you how much distance something covers in a given time. Motion is one of the first things physics ever tried to measure, and it starts with a single tidy equation.
Speed: distance over time
The equation you must know cold is:
Worked example: a car travels 100 m in 5 seconds. Write the equation first, then substitute: speed = distance ÷ time = 100 ÷ 5 = 20 m/s. Always keep the units lined up — metres and seconds give you m/s.
Speed vs velocity
People use “speed” and “velocity” interchangeably in everyday talk, but physics is fussier. Velocity is speed in a stated direction — for example, 20 m/s north. Two cars could travel at the same speed yet have different velocities if they head different ways. Think of speed as the number on the speedometer, and velocity as that number plus a compass direction.
Acceleration: how quickly velocity changes
Acceleration measures how fast an object’s velocity is changing — speeding up, slowing down or turning.
Worked example: a cyclist speeds up from 2 m/s to 10 m/s in 4 seconds. Change in velocity = 10 − 2 = 8 m/s. Acceleration = 8 ÷ 4 = 2 m/s². Notice the extra “per second” — acceleration is a change in speed every second.
Reading a distance–time graph
A distance–time graph plots how far an object has travelled against time. The steepness (gradient) of the line tells you the speed:
| Line on the graph | What it means |
|---|---|
| Steep line | Moving fast |
| Gentle slope | Moving slowly |
| Flat (horizontal) line | Stationary — not moving |
Memory hook: for speed, remember “Distance Steve Travels” — Distance = Speed × Time, so Speed = D ÷ T. On a graph: steeper = faster, flat = parked.