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How can we describe motion?

Velocity-time graphs

You should be able to explain velocity-time graphs for objects moving with a constant velocity or a changing velocity.

Background information

The velocity of an object is its speed in a particular direction. This means that two cars travelling at the same speed, but in opposite directions, have different velocities. One velocity will be positive, and the velocity in the other direction will be negative.

The vertical axis of a velocity-time graph is the velocity of the object and the horizontal axis is the time taken from the start.

Features of the graphs

When an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a steadily increasing velocity, or a steadily decreasing velocity, the line on the graph is straight, but sloped. The diagram shows some typical lines on a velocity-time graph.

time (s) on x axis, velocity (m/s) on y axis

Speed - time graph

The steeper the line, the more rapidly the velocity of the object is changing. The blue line is steeper than the red line because it represents an object that is increasing in velocity much more quickly than the one represented by the red line.

Notice that the part of the red line between 7 and 10 seconds is a line sloping downwards (with a negative gradient). This represents an object that is steadily slowing down.

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