Lesson focus: A fair speed investigation coordinates distance and time measurements and treats uncertaintyHow far a measurement could be from the true value, such as "25 cm, give or take 1 cm". honestly.
Plan and evaluate a trolley investigation with repeatDoing the same measurement again in the same way, to check how close the results are., control variables and suitable timing.
Build on earlier learning
Motion graphs: distance and timeGraphing and gradient support analysis of speed measurements.
Practical
Measuring speed on a ramp
- Mark a distance on a ramp and release a trolley from the same point each time.
- Time the trolley over the distance, or use light gates.
- Speed = distance ÷ time; repeat and calculate a mean.
- Use the model on this page to see how speed shows on a distance–time graph.
Record: Distances, times and mean speed; a distance–time graph.
Distance–time graph
Set the speed for each 10 s stage and move the time slider. A steeper line means a greater speed; a flat line means stationary.
Controls
You will be successful when you can…
- Define the independent variable.
- Measure distance and time suitably.
- Use repeats and a mean.
- Explain why a ramp trolley accelerates.
Words that carry the science
- light gate
- A sensor that times interruption of a light beam.
- repeat
- Another measurement under the same intended conditions.
- mean
- Sum of valid repeats divided by their number.
- uncertainty
- A reasonable interval around a measurement.
- acceleration
- Rate of change of velocity.
Define the measured speed
01Define the measured speed
Timing a trolley over a measured track length gives average speedThe total distance travelled divided by the total time taken. for that interval. A light gateA sensor that times how long something takes to pass through a beam of light. and card estimate speed over a much shorter interval.
A trolley down a ramp accelerates, so distance ÷ time is not necessarily its final speed.
02Use control variables for a fair comparison
When ramp height changes, keep trolley, release point, surface and timed section consistent. Release without pushing.
Measure height vertically and secure the track. Provide a clear run-off and safe trolley stop.
03Repeats reveal variation
Record raw times, repeatDoing the same measurement again in the same way, to check how close the results are. each condition, inspect anomalies and calculate a justified meanAn average found by adding up all the values and dividing by how many values there are..
Reaction time can dominate a short interval. Use light gates or a longer interval to reduce percentage timing uncertaintyHow far a measurement could be from the true value, such as "25 cm, give or take 1 cm"..
Process repeated times
A trolley covers 1.50 m in 1.21, 1.18 and 1.20 s.
- Check all values are plausible.
- Mean = 3.59 ÷ 3 = 1.196… s.
- Round sensibly to 1.20 s.
- Speed = 1.50 ÷ 1.20 = 1.25 m/s.
How to make the reasoning visible: Mean average speed is 1.25 m/s.
Trolley and ramp
Use these illustrative results to calculate the speed at each height.
| Height / cm | t₁ / s | t₂ / s | t₃ / s | Mean / s |
|---|---|---|---|---|
| 5 | 2.42 | 2.38 | 2.40 | 2.40 |
| 10 | 1.82 | 1.79 | 1.81 | 1.81 |
| 15 | 1.51 | 1.49 | 1.50 | 1.50 |
| 20 | 1.32 | 1.29 | 1.31 | 1.31 |
Calculate speeds over 1.50 m.
State the pattern and one limitation.
Check your understanding
Answer from memory, then check against the suggested answer.
What speed does distance ÷ interval time give?
Why repeat?
Name a control.
Why is hand timing 0.20 m weak?
How can two light gates test acceleration?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student releases a trolley by hand, times it over 0.50 m once with a stopwatch, and calls distance ÷ time “the trolley’s speed”.
Identify three problems with the method.
(3)What does distance ÷ time actually give? Suggest one improvement.
(2)Total for this question: 5 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., link each limitation to its effect and an improvement.