IGCSE Physics / Movement and position
Velocity-time graphs revision notes
Edexcel International GCSE Physics (4PH1), chapter 1: Movement and position, specification point 1.7, 1.8. Free lesson.
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A velocity-time graph shows how an object's velocity changes over time. Its gradient gives the acceleration, and its shape must be read using the label on the vertical axis.
How to read a velocity-time graph
Time is on the horizontal axis in seconds, and velocity is on the vertical axis in metres per second. Velocity shows how fast an object is moving and in which direction. The line's shape tells you how its motion changes. A line sloping up shows increasing velocity, so the object is accelerating. A horizontal line shows constant velocity. The object is still moving unless the line is at zero velocity. A line sloping down shows decreasing velocity, so the object is decelerating. A steeper slope means a greater acceleration or deceleration.
How to find acceleration from the gradient
The gradient of a velocity-time graph is the acceleration. Find the change in velocity and divide it by the time taken. Read both values from the axes. A downhill line gives a negative acceleration because the velocity is falling. Acceleration is measured in metres per second squared, written m/s².
What a horizontal line means
A horizontal line on a velocity-time graph means the velocity is not changing, so acceleration is zero. It does not mean the object has stopped. The object is stopped only when the line lies along the time axis, where velocity is zero. A horizontal line above or below the time axis means constant velocity. A line along the time axis means zero velocity, so the object is stopped.
How velocity-time graphs differ from distance-time graphs
The same line shape can describe different motion on different graphs. Check the vertical-axis label first, because it tells you what the gradient and the area under the line mean. On a distance-time graph, a horizontal line means stationary. On a velocity-time graph, it means constant velocity. A straight line sloping up shows constant speed on a distance-time graph, but constant acceleration on a velocity-time graph. The gradient gives speed on a distance-time graph and acceleration on a velocity-time graph. The area under a velocity-time graph gives distance travelled. The area under a…