The formula
a = (v − v₀) / t
| Symbol | Quantity | Unit |
|---|---|---|
| a | Acceleration | m/s² |
| v | Final velocity | m/s |
| v₀ | Initial velocity | m/s |
| t | Time | s |
What it means
Acceleration is the change in velocity divided by the time it takes. Its units are metres per second per second, which sounds odd until you read it as 'the speed changes by this many m/s every second'.
How to work it out by hand
- Subtract the initial velocity from the final one
- Divide by the time taken
- A negative answer means it is slowing down
What is worth knowing
Acceleration is a vector, so a negative value does not always mean braking: it means the change points the other way. A car reversing and speeding up has negative velocity and negative acceleration. The one everyone knows is gravity, 9.81 m/s², which is why a dropped object gains about 10 m/s of speed every second. Note also that turning at constant speed is still acceleration, because the direction is changing even though the number on the speedometer is not.