Lesson focus: A motion investigation turns position and time measurements into evidence while controlling release, route and measurement uncertaintyHow far a measurement could be from the true value, such as "25 cm, give or take 1 cm"..
Design and evaluate a trolley investigation and derive speed or acceleration from timed position data.
Build on earlier learning
Investigating speedYear 8 planned a trolley speed investigation. This lesson adds timing gates, velocity change and uncertainty.
Practical
Investigating motion
- Release a trolley down a ramp through two light gates.
- Record the speed at each gate and the time between them.
- Acceleration = change in velocity ÷ time.
- Use the model on this page to build distance–time graphs.
Record: Speeds, times and acceleration.
Accelerating trolley
Set the acceleration and move the time slider. The curve gets steeper as the trolley speeds up.
Controls
You will be successful when you can…
- Identify the independent, dependent and control variables.
- Choose suitable timing equipment.
- Process position–time data.
- Evaluate uncertainty and repeats.
Words that carry the science
- light gate
- A sensor that records when an interrupt card blocks a light beam.
- fiducial marker
- A fixed reference mark used to define a measurement position.
- reaction time
- Delay between detecting an event and responding to it.
- systematic error
- A consistent measurement offset that shifts results in one direction.
- random error
- Unpredictable variation causing repeated values to scatter.
- uncertainty
- A quantified range expressing reasonable doubt in a measurement.
A clear question determines the method
01A clear question determines the method
To test how ramp height affects trolley acceleration, change ramp height, measure motion and control trolley, ramp surface, release position and timing arrangement. Release without a push.
Use distance markers with video or light gateA sensor that times how long something takes to pass through a beam of light.. A handheld stopwatch is usually poor for short intervals because reaction timeThe time it takes you to react to something, such as a driver seeing danger and pressing the brake. is a large fraction of the measured time.
02Sensors still need careful geometry
A light gate measures blocking time. With an interrupt card of known length, speed at the gate is card length divided by blocking time.
Measure card length in metres and ensure it passes squarely through the beam. A wrong card length produces a systematic scale error.
03Repeats reveal variation, not truth automatically
Repeat independent releases, calculate means and inspect spread. Repeats reduce random uncertaintyHow far a measurement could be from the true value, such as "25 cm, give or take 1 cm". in a mean but do not correct a biased zero, tilted ruler or wrong calibration.
Plot processed values with labelled axes and units. Avoid joining scattered measurements dot-to-dot when a best-fit trend is more appropriate.
Calculate gate speed
A 0.050 m interrupt card blocks a light gate for 0.020 s.
- Use speed = card length ÷ blocking time.
- Substitute 0.050 ÷ 0.020.
- Calculate 2.5.
- Attach the unit m s⁻¹.
How to make the reasoning visible: The speed at the gate is approximately 2.5 m s⁻¹ over the short interval represented by the card.
Motion investigation
A trolley passes two gates 0.80 m apart after being released from the same ramp position.
| Ramp height / cm | Speed at gate 1 / m s⁻¹ | Speed at gate 2 / m s⁻¹ | Time between / s |
|---|---|---|---|
| 5 | 0.42 | 0.66 | 1.50 |
| 10 | 0.60 | 0.98 | 1.20 |
| 15 | 0.74 | 1.28 | 1.00 |
| 20 | 0.86 | 1.55 | 0.88 |
Graph skills: SLAPUK and choosing a scaleMotion results: choose your scale before you plot.
Calculate mean acceleration for the 15 cm row.
Why keep release position fixed?
Check your understanding
Answer from memory, then check against the suggested answer.
Why are light gates useful?
What do repeats reveal?
Can repeats remove systematic error?
Every recorded speed is 10% too high because the card length was entered wrongly. What error is this?
Why is gate speed not perfectly instantaneous?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student times a trolley rolling 20 cm down a ramp with a handheld stopwatch, one trial at each ramp height.
Evaluate the method. Identify the weaknesses.
(3)Suggest improvements.
(3)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., pair each limitation with its effect and a matched improvement.