Deceleration
If the value obtained for acceleration is positive, it signals an increase in velocity. If it is negative, it says the velocity is decreasing.
A case where velocity decreases
Suppose an object moving along a straight path starts with a velocity of 12 m s−1, which then changes as shown.
| Time t (s) | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|
| Velocity v (m s−1) | 12 | 9 | 6 | 3 | 0 |
The calculation
acceleration = (0 − 12) m s−1 ÷ 4 s = −3 m s−2
Here the acceleration comes out negative. It says the velocity decreases by 3 m s−1 every second.
Deceleration
When an object's velocity is decreasing, its acceleration takes a negative value. A negative acceleration is called a deceleration, or retardation.
If an object's acceleration is −3 m s−2, its deceleration is 3 m s−2. The sign drops away because the word "deceleration" already carries that meaning.
The signs together
| Sign of acceleration | What velocity does | Name |
|---|---|---|
| Positive (+) | Increases | Acceleration |
| Negative (−) | Decreases | Deceleration |
| Zero (0) | Stays the same | Uniform velocity |