Measuring average speed
Key idea: Measure the distance between two lines and time the same point on an object crossing each. Repeat the timings, find the mean time, and divide the distance by it to find average speed.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Measure distance and time for the same journey.
- Include the stop in the whole-journey calculation.
- Find the trolley’s average speed in metres per second.
- Explain why timing different car edges mismatches distance and time.
- Plan a repeatable toy-car speed measurement.
- Compare the repeat timings and investigate unusual results.
- Calculate mean time and the average-speed estimate.
- Explain a shared mistake that repeats cannot correct.
- Compare average speeds with and without the unusual timing.
- Explain what to check before excluding a reading.
On this page
You stopped at:
ContinueTo find an average speed you need two measurements that belong together: a distance, and the time for that same journey. Clear events for starting and stopping the stopwatch make the result trustworthy.
Average speed
With distance in metres and time in seconds, average speed is in metres per second (m/s). For a whole journey, the total time includes any stops. The average speed is not necessarily the speed at the end.
To measure it, mark a start line and a finish line and measure the distance between them with a measuring tape. Release the object the same way each time. Start and stop the stopwatch as the same point on the object crosses each line. Repeat the timing, find the mean time, and divide the distance by it.
Worked example 1
Include a stop in the whole journey
Problem
A toy car travels 3.0 m, stops for 2.0 s and then travels another 2.0 m. The whole journey takes 10.0 s. Find its average speed over the whole journey.
Show full solution
Match distance and time
Method
Use all the distance and all the elapsed time for the same journey.
Reason
Removing the stop would describe a different time interval from the one asked for.
Working
Answer
Total distance = 3.0 m + 2.0 m = 5.0 m; total time = 10.0 s.
Calculate and check
Method
Divide total distance by total time.Reason
Metres divided by seconds gives m/s. Multiplying the answer by the journey time recovers the distance.
Working
Answer
Average speed = 5.0 m ÷ 10.0 s = 0.50 m/s. Check: 0.50 m/s × 10.0 s = 5.0 m.
Quick check
Guided practice 2
Determine a trolley’s average speed
Problem
A trolley travels 3.6 m in 4.5 s. Determine its average speed.
Calculate the average speed
Hints
Hint 1
choose the relationship
Average speed uses the total measured distance and total measured time.
Hint 2
substitute with units
Divide 3.6 m by 4.5 s.
Show solution step by step
Select the measured quantities
Method
Use average speed = total distance ÷ total time.Reason
The question describes the complete measured journey rather than the speed at one instant.
Working
Answer
average speed = 3.6 m ÷ 4.5 sCalculate and record the unit
Reason
Metres divided by seconds gives metres per second.Working
Answer
average speed = 0.80 m/s
Quick check
Spot the mistake 3
Time the same point on the car
Problem
Start and finish lines are 2.00 m apart. A learner starts timing when the front of a car reaches the start line and stops when its rear reaches the finish line. They divide 2.00 m by this time. Explain the problem and improve the method.
Explain your reasoning before revealing the solution
Show solution step by step
Identify the mismatched interval
Method
Notice that different points on the car trigger the start and stop.
Reason
The front has travelled farther than 2.00 m when the rear reaches the finish line, so the recorded time does not match the measured distance.
Working
Answer
The interval lasts until the whole car has passed the finish line.
Use a consistent reference
Method
Start and stop timing as the same point, such as the front, reaches each line.
Reason
The chosen point then travels exactly the measured start-to-finish distance.
Working
Answer
Front at start line → start timer; front at finish line → stop timer. Repeat using the same release.
Common mistake
Exam-style question 1
Plan a reliable average-speed measurement
Practical planning question
Describe how to determine the average speed of a toy car over a marked track using a measuring tape and digital stopwatch. Include one way to improve confidence in the result. Use the four-point self-check after answering.
Describe the practical method
Show solution step by step
Measure a defined distance
1 markMethod
Mark start and finish lines and measure the distance between them with the tape.
Reason
A defined distance is needed for the average-speed relationship.
Working
Answer
Record the distance in metres.Measure the matching time
1 markMethod
Time the same point on the car crossing both lines.
Reason
Clear start and stop events make trials comparable.Working
Answer
Record the journey time in seconds.Repeat and calculate
2 marksMethod
Repeat with the same release, find a mean time and divide distance by mean time.
Reason
Repeats reveal timing variation and reduce the influence of one unusually early or late response.
Working
Answer
average speed = measured distance ÷ mean time, in m/s
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Self-mark the distance, timing, repeat and calculation points separately.
Repeated timings
Put the quantity and unit in each table heading, and include every repeat.
| Trial | Distance / m | Time / s | Average speed / m/s |
|---|---|---|---|
| 1 | 2.00 | 3.9 | 0.51 |
| 2 | 2.00 | 4.1 | 0.49 |
| 3 | 2.00 | 4.0 | 0.50 |
The mean time is (3.9 + 4.1 + 4.0) ÷ 3 = 4.0 s, giving an average speed of 2.00 m ÷ 4.0 s = 0.50 m/s.
A result very different from the others is an anomaly. Look for a reason, such as a late press of the stopwatch, and repeat that trial. Keep the unusual reading in the table and record the investigation. A new repeat cannot tell you why the earlier reading differed. Exclude a reading from an estimate only with a justified, recorded reason, such as an observed stopwatch mistake; otherwise retain it and report the variation.
Try it yourself 4
Decide whether a mean time is trustworthy
Problem
A car is released in the same way over 2.40 m. Timings are 3.1 s, 3.2 s and 3.3 s. Find the mean time and the average-speed estimate.
Calculate the mean time and speed
Show the steps Optional working
Show the steps Optional working
Hints
Hint 1
Use a typical time
Find the mean of all three comparable times.
Hint 2
Think about a shared mistake
Could every trial use the same incorrect finish event?
Show solution step by step
Combine comparable trials
Method
Find the mean time, keeping the track distance fixed.
Reason
The trials describe the same setup and release, so their variation can be used to estimate a typical journey time.
Working
Answer
Mean time = (3.1 s + 3.2 s + 3.3 s) ÷ 3 = 3.2 s. Average-speed estimate = 2.40 m ÷ 3.2 s = 0.75 m/s.
Check the method as well as the repeats
Method
Use repeats to assess variation, while still checking the timing method and distance.
Reason
A consistent late trigger or an incorrectly measured track can affect every trial. A mean does not remove a shared mistake.
Working
Answer
The close readings show little variation between repeats. Check that the same point on the car triggers both timings, start/stop events and track measurement before trusting the result.
Quick check
Check your understanding 1 Check a shared timing mistake
A learner says agreement between the 3.1, 3.2 and 3.3 s readings means the timing has no error. Explain a mistake all trials could share and what to check.
Show answer
All trials could stop on the rear edge after starting on the front edge, or use the same wrongly measured track. Close readings show little repeat-to-repeat variation but do not correct a shared error. Check the measured distance and use the same point on the car at both start and finish lines.
Check your understanding 2 An unusual timing
A trolley is timed over 1.50 m four times: 2.4 s, 2.6 s, 3.9 s and 2.5 s. Which result is unusual? Calculate the mean time and average speed using all four readings, and using only the three close readings, as you would if the student saw a timing mistake in that trial. Use unrounded mean times for the speed calculations.
Show the steps Optional working
Show the steps Optional working
Show the steps Optional working
Show the steps Optional working
Show answer
3.9 s is much longer than the others. It could be a late stop press or a trolley that really moved more slowly. Check how the trolley was released and timed, repeat the trial and record what happened. Keeping all four readings gives mean time = 11.4 ÷ 4 = 2.85 s and an estimate of 1.50 ÷ 2.85 ≈ 0.53 m/s. If an observed timing mistake justifies excluding 3.9 s, the three remaining readings give mean time 2.5 s and an estimate of 0.60 m/s. Only leave out a reading for a recorded reason, and keep it in your results table.
Check your understanding 3 Investigate before excluding a reading
What should the student check about the 3.9 s reading before deciding whether to leave it out of the speed estimate?
Show answer
The 3.9 s could come from a stopwatch mistake or a genuinely slower run. Check the release and timing events, repeat and record what happened. Keep every original reading in the table. Exclude it from an estimate only for a recorded reason such as an observed timing mistake; a new repeat alone cannot identify the earlier cause.
Common mistakes
- Leaving the time spent stopped out of the total time for a whole journey.
- Starting the stopwatch with the front of the car and stopping it with the back.
- Using a distance in centimetres with a speed in m/s.
- Deleting an anomalous reading without checking or recording it.
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Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: