Velocity–Time Graphs (SQA National 5 Physics): Revision Note

Velocity-time graphs

• Speed-time graphs and velocity-time graphs are used to describe the motion of an object

  • Speed, or velocity, is always plotted on the vertical axis (y-axis) as it is the dependent variable

  • Time is always plotted on the horizontal axis (x-axis) as it is the independent variable

• A speed-time graph shows how the speed of an object varies with time

  • Since speed is a scalar quantity, the values of speed will always be positive

Speed-time graph for different objects

• A velocity-time graph shows how the velocity of an object varies with time

  • This looks very similar to a speed-time graph, but since velocity is a vector quantity, the values of velocity may be positive or negative to indicate direction

Velocity-time graph for a ball thrown upwards

Interpreting velocity-time graphs

• The slope of the line on a velocity-time graph represents the magnitude of acceleration

  • The steeper the slope, the larger the acceleration

    • The object's speed changes very quickly

  • The shallower the slope, the smaller the acceleration

    • The object's speed changes very gradually

  • A positive slope indicates increasing velocity

    • The object is accelerating

  • A negative slope indicates decreasing velocity

    • The object is decelerating

  • A horizontal (flat) line means the acceleration is zero

    • The object is moving with a constant velocity

Constant acceleration and constant velocity on a velocity-time graph