ABLESpeed, Velocity & Acceleration
Speed
Speed is the distance travelled per unit time. It is a scalar quantity, meaning it only has magnitude and no direction. The SI unit of speed is metres per second (m/s).
Velocity
Velocity is the displacement per unit time. Unlike speed, velocity is a vector quantity — it has both magnitude and direction. An object moving in a circle at constant speed has a changing velocity because its direction is constantly changing.
Acceleration
Acceleration is the rate of change of velocity. It is a vector quantity. Positive acceleration means speeding up; negative acceleration (deceleration) means slowing down.
Distance-Time Graphs
On a distance-time graph, the gradient represents speed. A straight line means constant speed. A curved line means changing speed (acceleration). A horizontal line means the object is stationary.
Velocity-Time Graphs
On a velocity-time graph, the gradient represents acceleration. The area under the graph represents the distance travelled. A horizontal line means constant velocity (zero acceleration).
Key Points
- Speed is scalar; velocity is a vector (has direction)
- Acceleration = change in velocity ÷ time
- Gradient of a distance-time graph = speed
- Gradient of a velocity-time graph = acceleration
- Area under a velocity-time graph = distance
Exam Tips
- Always check units — convert km/h to m/s by dividing by 3.6
- When asked about velocity vs speed, mention direction
- Show your working clearly when using v = d/t or a = (v-u)/t