Two words that sound almost identical — speed and velocity — actually describe two different things, and mixing them up is one of the most common mistakes in physics.
On 16 August 2009, Usain Bolt ran 100 metres in 9.58 seconds — a new world record. Sports commentators call that "speed." But a physicist would want to know one more thing before calling it complete: which direction was he running in? That single extra detail is the entire difference between speed and velocity, and it's one of the most useful distinctions in all of physics.
Before speed and velocity make sense, two related ideas need separating: distance and displacement. Distance is the total length of the path an object travels — it only cares about how much ground was covered. Displacement is the straight-line distance between the starting point and the ending point, along with the direction — it only cares about where you ended up relative to where you began.
| Concept | What It Measures | Has Direction? | Example |
|---|---|---|---|
| Distance | Total path length travelled | No | Running 400m around a track |
| Displacement | Straight-line change in position | Yes | 0m (if you end up back where you started) |
Running a full lap of a 400m track (left) covers real distance but ends with zero displacement. Walking directly from Point A to Point B (right) — distance and displacement are the same straight line.
This is where the confusion usually happens. Speed tells you how fast something is moving — distance covered per unit of time. Velocity tells you the same thing, plus the direction of travel. A car going "60 km/h" has a speed. A car going "60 km/h north" has a velocity.
| Speed | Velocity | |
|---|---|---|
| Formula | Distance ÷ Time | Displacement ÷ Time |
| Direction included? | No | Yes |
| Type of quantity | Scalar | Vector |
| Example | "40 km/h" | "40 km/h, heading east" |
In physics, quantities that have only a size (like speed, distance, mass, or temperature) are called scalar quantities. Quantities that have both size and direction (like velocity, displacement, and force) are called vector quantities. This distinction matters more than it might seem — two cars both moving at 40 km/h but in opposite directions have the same speed, but completely different velocities.
Speed is calculated with one of the simplest formulas in physics:
Speed = Distance ÷ Time
If a cyclist covers 30 kilometres in 2 hours, their average speed is 30 ÷ 2 = 15 km/h.
This formula rearranges in two useful ways: Distance = Speed × Time (used to find how far something travelled), and Time = Distance ÷ Speed (used to find how long a journey will take). All three versions are the same relationship, just solved for a different unknown.
Not all motion happens at a constant rate. Physics separates motion into a few key types based on how speed changes over time.
| Type of Motion | Description | Example |
|---|---|---|
| Uniform motion | Constant speed, no change over time | A car on cruise control on an empty highway |
| Non-uniform motion | Speed changes over time (speeding up or slowing down) | A car in city traffic |
| Acceleration | Speed (or direction) increasing over time | A sprinter leaving the starting blocks |
| Deceleration | Speed decreasing over time | A car braking at a red light |
Even a car moving in a perfect circle at a totally constant speed is technically accelerating in physics terms — because its direction is constantly changing, and velocity depends on direction. This is a good example of how physics definitions can be more precise than everyday language.
One of the most useful tools in studying motion is the distance-time graph — and once you know how to read one, a huge amount of information becomes instantly visible.
On a distance-time graph, the steepness (slope) of the line is the speed. A flat line means the object has stopped — distance isn't increasing even though time still is.
The key rule: the steeper the line, the faster the speed. A flat, horizontal line means the object isn't moving at all, no matter how far to the right the graph continues — time is passing, but distance isn't increasing. This makes distance-time graphs a quick way to compare journeys at a glance, without reading a single number.
Usain Bolt's 9.58-second 100m world record (Berlin, 2009) works out to an average speed of about 37.58 km/h across the whole race — but his laser-measured top speed, reached between the 60m and 80m mark, was about 44.72 km/h. The difference exists because he wasn't moving at a constant speed: he accelerated from a standstill, then reached his fastest pace partway through, matching the "steeper line" idea on a distance-time graph.
Speed is easiest to appreciate side by side. Here's how some familiar — and some extreme — speeds compare.
| Moving Object | Approximate Speed |
|---|---|
| Average walking pace | 5 km/h |
| Usain Bolt's 100m world record (average) | 37.58 km/h |
| Usain Bolt's top recorded speed | 44.72 km/h |
| Typical car on a highway | 100–120 km/h |
| Commercial jet aircraft (cruising) | ~900 km/h |
| Speed of sound (in air, at sea level) | ~1,235 km/h (343 m/s) |
| Speed of light (in a vacuum) | ~1,079,000,000 km/h (about 299,792 km/s) |
Notice how much of a jump each row is: a jet is roughly 20 times faster than a highway car, sound is roughly 10 times faster again than a jet, and light is faster than sound by a margin so large it barely fits in comparison — nothing in the universe travels faster than light in a vacuum.
When you see distant lightning before you hear the thunder, you're witnessing the speed gap between light and sound directly. Light from the lightning strike reaches your eyes almost instantly, while the sound has to travel through air at roughly 343 metres per second — slow enough that counting the seconds between the flash and the sound (then dividing by three) gives a rough estimate of how many kilometres away the storm is.
Speed and velocity look like a small technical distinction, but they capture something physics cares about deeply: precision. "How fast" and "how fast, in what direction" are genuinely different questions with genuinely different answers — and once this distinction clicks, ideas from later physics, like force and acceleration, become much easier to follow, since they all depend on getting direction right.
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