Speed-time graphs

The speed-time graph below illustrates how the speed of an object changes with time.

Speed against time graph of uniform acceleration (steadily increasing line), steady speed (flat line), further uniform acceleration and finally uniform deceleration (line steadily decreasing to zero)

A horizontal line on a speed-time graph represents a steady speed. This can also be called a constant or uniform speed.

A sloping line on a speed-time graph represents an acceleration. The sloping line shows that the speed of the object is changing. The object is either speeding up or slowing down.

  • The steeper the slope of the line the greater the acceleration.
  • If the line slopes upward from left to right, this means the object is speeding up. Its acceleration will be positive.
  • If the line slopes downward from left to right on the graph, this means that the object is slowing down. Its acceleration will be negative.