Speed, velocity and acceleration

Speed is the distance travelled per unit time and is a scalar quantity, measured in metres per second. Velocity is speed in a stated direction, so it is a vector.

Acceleration is the rate of change of velocity, measured in m/s². A body slowing down has negative acceleration, called retardation or deceleration.

On a velocity–time graph, the slope gives the acceleration and the area under the graph gives the distance travelled. A horizontal line means constant velocity and zero acceleration.