Velocity and acceleration · Grade 9
Velocity is speed in a given direction, and acceleration is how quickly velocity changes. Velocity–time graphs are powerful: their gradient gives the acceleration and the area underneath gives the distance travelled.
Key facts
- Acceleration a = (v − u)/t, measured in m/s². Slowing down is a negative acceleration (deceleration).
- Velocity–time graph: gradient = acceleration; area under the line = distance travelled.
- A horizontal line on a v–t graph means constant velocity; a line sloping down means slowing down.
- Objects falling freely near Earth accelerate at about 9.8 m/s².
- Average speed from a v–t graph = total area ÷ total time.