Plot speed-time graphs and identify motion states
Read speed from height and distinguish rest, constant speed, acceleration and deceleration.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Plot time-value coordinate pairs.
- Distinguish speed from distance.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the vertical quantity on a speed–time graph?
What you will learn
- Plot a speed–time record with labelled axes.
- Interpret horizontal and rising or falling sections.
Height is speed on these axes
A speed–time graph has time on the horizontal axis and speed on the vertical axis. The points (0 s, 0 m/s), (4 s, 8 m/s), (7 s, 8 m/s), (9 s, 0 m/s) joined by straight sections show speeding up, then constant speed, then slowing to rest. The vertical height at six seconds is 8 m/s.
A horizontal line at positive speed means continuing motion at constant speed. A horizontal line on the zero-speed axis means rest. These interpretations differ from a distance–time graph, where any horizontal cumulative-distance segment means rest. Check the axis quantity before describing motion.
Interpret changes over time
A rising speed line shows acceleration in the qualitative sense of speeding up; a falling one shows deceleration. A steeper change means speed changes more quickly. The numerical rate of change and units m/s² belong to the separately labelled Supplement lessons.
A single speed–time graph does not supply the direction of motion, so it cannot distinguish two journeys with equal speed histories but different directions. Plotting line segments between supplied points assumes the stated change is linear in each interval; isolated speed measurements alone do not establish the exact motion between them.
Sources for this lesson
Pause and explain
What does the horizontal 4–7 s section at 8 m/s mean?
Worked example
Interpret the straight sections through (0,0), (4,8), (7,8), (9,0) in s and m/s.
Show the worked solution
- 0–4 s: speed increases from zero to 8 m/s.
- 4–7 s: speed stays at 8 m/s.
- 7–9 s: speed falls to zero, ending at rest.
Answer Accelerates; constant speed; decelerates to rest.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Plot straight speed sections through (0,0), (4,8), (7,8), (9,0). Describe each interval.
Give me a hint
Read rising, horizontal and falling speed values.
Compare my reasoning
- Label time in seconds and speed in m/s.
- Plot and join the stated straight sections.
- Interpret acceleration, constant speed and deceleration to rest.
Accelerates 0–4 s; constant 4–7 s; decelerates 7–9 s.
Look for these in your work
- Label the axes.
- Interpret positive horizontal speed correctly.
A speed graph is horizontal at 6 m/s for five seconds, then at zero for two seconds. Describe both states.
Give me a hint
Horizontal means unchanged speed; its height determines whether motion continues.
Compare my reasoning
- The first interval has speed 6 m/s.
- That is constant non-zero motion.
- The second interval has zero speed and is rest.
Constant speed for five seconds, then rest for two seconds.
Look for these in your work
- Use the vertical value.
- Distinguish constant speed from rest.
A student labels every horizontal graph section as rest. Explain how to correct this for distance–time and speed–time graphs.
Give me a hint
First identify what the vertical axis measures.
Compare my reasoning
- Horizontal cumulative distance means no additional path is travelled.
- Horizontal positive speed means a fixed non-zero rate.
- Only a zero-speed horizontal section means rest on the speed graph.
Use the vertical quantity; horizontal positive speed is continued motion.
Look for these in your work
- Compare both graph types.
- Include the zero-speed condition.
Common mistakes
- Positive horizontal speed is motion, not rest.
- A speed graph does not specify direction.
- Do not call the graph's vertical reading a distance.
What can you explain now?
What does a horizontal speed–time line at 8 m/s mean?
Compare with the explanation
Constant speed of 8 m/s.
The speed remains positive and unchanged, so the object keeps moving.
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Make it stick
Draw matching distance–time and speed–time graphs for constant-speed travel followed by a stop.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.