Motion and Graphs
Translate graph shape into motion before using a formula.
Motion and Graph Interpretation
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Read the axes first: slope and area do different jobs
A position–time graph tells where an object is. Its slope is velocity. A velocity–time graph tells how velocity changes. Its slope is acceleration, while its signed area is displacement.
Position–time
Steeper slope means greater speed; horizontal means rest.
Velocity–time
Horizontal means constant velocity; area gives displacement.
Name the axes before reading the line
Problem: A position–time line rises from 0 m to 20 m in 5 s.
Recognize: The vertical axis is position, so slope means velocity.
Do it fast: (20−0)/(5−0)=4 m/s.
Why it works
The same-looking line means different things on different axes. Naming the axes prevents the most common graph error.
Five forms you should recognize
From (2 s, 5 m) to (6 s, 17 m):
v=(17−5)/(6−2)=3 m/sThe straight line means this velocity is constant.
A horizontal line at 8 m/s for 5 s forms a rectangle:
displacement=(8 m/s)(5 s)=40 mMoving from +10 m to −2 m in 4 s:
v=[−2−10]/4=−3 m/sThe negative sign describes direction.
A horizontal position–time line means the position is not changing, so velocity is zero. The line may be above or below position zero.
One complete lap ends at the starting point. The path length is the lap distance, but displacement is zero because initial and final positions match.
Check before you commit
- Confusing graph height with slope
- Calling every rising line acceleration
- Forgetting signed area
- Mixing distance and displacement
- Ignoring units
- Treating a curved position graph as constant velocity
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Motion and Graphs FAQ
Does a horizontal line always mean rest?
Only on a position–time graph. On a velocity–time graph, it means constant velocity.
Can velocity be negative?
Yes. The sign indicates direction relative to the chosen positive axis.
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