Describing the motion of an object is occasionally hard to do with words. Sometimes graphs help make motion easier to picture, and therefore understand.
Remember:
• Motion is a change in position measured by distance and time.
• Speed tells us the rate at which an object moves.
• Velocity tells the speed and direction of a moving object.
• Acceleration tells us the rate speed or direction changes.
DISTANCE-TIME GRAPHS
Plotting distance against time can tell you a lot about motion. Let's look at the axes:
Time is always plotted on the X-axis (bottom of the graph). The further to the right on the axis, the longer the time from the start.
Distance is plotted on the Y-axis (side of the graph). The higher up the graph, the further from the start.
If an object is not moving, a horizontal line is shown on a distance-time graph.
Time is increasing to the right, but its distance does not change. It is not moving. We say it is At Rest.
If an object is moving at a constant speed, it means it has the same increase in distance in a given time:
Time is increasing to the right, and distance is increasing constantly with time. The object moves at a constant speed.
Constant speed is shown by straight lines on a graph.
A steeper line indicates a larger distance moved in a given time. In other words, higher speed.
Both lines are straight, so both speeds are constant.
Let’s look at two moving objects:
Both of the lines in the graph show that each object moved the same distance, but the steeper dashed line got there before the other one:
The line on this graph is curving upwards. This shows an increase in speed, since the line is getting steeper:
In other words, in a given time, the distance the object moves is change (getting larger). It is accelerating.
Graphs that show acceleration look different from those that show constant speed.
Summary:
A distance-time graph tells us how far an object has moved with time.
• The steeper the graph, the faster the motion.
• A horizontal line means the object is not changing its position - it is not moving, it is at rest.
• A downward sloping line means the object is returning to the start.
