ACARA v9 CONTENT DESCRIPTION “plan and conduct valid, reproducible investigations to answer questions and test hypotheses, including identifying and controlling for possible sources of error and, as appropriate, developing and following risk assessments, considering ethical issues, and addressing key considerations regarding heritage sites and artefacts on Country/Place”
Builds on the valid, reproducible plan from earlier years and turns it on a Year 10 physics question. An investigation now has to be valid, measuring the quantity you actually intend with confounds controlled, and reproducible, written so another group repeats it and gets consistent results. Along the way you identify and control sources of error, follow a worked risk assessment, and act ethically and respectfully on Country and Place.
The scenario: does force control acceleration
A class tests Newton’s second law, the relationship written as force equals mass times acceleration. They let a small trolley run down a fixed ramp and time it between two gates to find its acceleration, then add a known pulling force from a hanging mass over a pulley and measure the acceleration again. The question is whether a larger force produces a larger acceleration in the way the law predicts, and the hypothesis is that doubling the force roughly doubles the acceleration when the moving mass is held the same.
Validity: measuring what you mean to measure
A valid investigation measures the intended quantity and nothing else. Here the intended effect is the applied force, so everything else that could change the acceleration must be held steady: the same total moving mass, the same ramp angle, the same track and surface, the same gate spacing, the same release point. The acceleration must also be the quantity actually computed from the two gate times, not just an average speed read off once. If the ramp angle creeps up or the mass changes between runs, a larger acceleration could be blamed on the force when the real cause was a steeper slope or a lighter trolley. Friction is the classic confound in motion work, so the same low-friction track is used throughout and a short coast test checks it is roughly constant. Controlling these confounds is what makes the comparison fair, and therefore valid.
Control the confounds so the force test is valid
You are testing whether a larger applied force gives a larger acceleration. The applied force is the one thing you change. Decide what to do with every other variable so the comparison stays valid.
On a fixed low-friction ramp your class times a trolley between two gates to find its acceleration, first with a small pulling force and then with a doubled force from a heavier hanging mass.
Variable being tested: The applied pulling force (a small force versus a doubled force) (this one we change)
The total moving mass, including the trolley and any load
The ramp angle and the low-friction track surface
The gate spacing and the release point of the trolley
The timing method and how each gate is triggered
Not a fair test yet: more than one thing is changing, so you could not tell which change caused the result. Hold every other variable the same.
Reproducibility: writing it so others get consistent results
Validity makes one comparison trustworthy; reproducibility makes the whole method shareable. To be reproducible the plan must pin down the quantities: the exact moving mass, the ramp angle, the gate spacing in centimetres, the hanging masses used for each force, the release point, and how many repeat runs to average. A method that just says roll a trolley down a slope and add some weight is too vague to repeat, and the next group would get scattered accelerations. A method that fixes every quantity and rule lets another class follow the same steps and reach consistent acceleration values, which is the real test of a claim about force and motion.
Identifying and controlling sources of error
Even a valid plan carries error, and a careful scientist names it rather than hiding it. Hand-started gates add reaction-time scatter, so an electronic trigger or one fixed triggering rule is used. Friction is never quite zero, so the same track is kept clean and a coast test estimates its size. Measuring the gate spacing once with a metre rule fixes a length error that would otherwise feed straight into every acceleration. Single runs can mislead, so several runs are timed at each force and averaged. Each of these is a source of error you identify and then control, so scatter from the method does not drown the real effect of force on acceleration.
Choose how many runs to time at each force setting
Each choice carries a benefit and a cost for how reliable and reproducible the acceleration is. Pick one to see the trade-off.
Your group must decide how many timed runs to do at the small force and at the doubled force. The choice affects both the time on the day and how trustworthy the averaged acceleration is.
Choose a response to see what is gained and what is given up.
The risk assessment
A fast, loaded trolley and a raised ramp mean a risk assessment is part of the plan, not a formality. You list each hazard and the control beside it: the ramp can slip, so it is clamped firmly to the bench; the trolley can shoot off the end, so a padded stop or catch box waits at the bottom and feet stay clear of the run-off zone; stacked masses can fall, so they are secured before each run and eye protection is worn near the hanging mass. Writing the controls down is also what lets the next group repeat the work safely, which ties risk management back to reproducibility.
Ethics and heritage on Country and Place
Running the ramp outdoors on a natural slope means recognising that the land is Country and Place. Some sites, rocks and artefacts hold cultural heritage significance for Aboriginal and Torres Strait Islander peoples. Grinding grooves, arrangements of stones, scarred trees and marked rocks can be culturally and legally protected. The ethical and lawful plan is to leave any such site, rock or artefact undisturbed, never remove or damage it, and seek permission and guidance from Traditional Owners before working in an area.
Sorting the plan: valid, reproducible, safe and respectful
Before timing a single run, check each planned action against the claim that this is a well-designed investigation: one that is valid, reproducible, safe, and respectful of heritage on Country and Place. Sort each action by whether it genuinely supports that claim or whether it does not belong in the plan at all.
Judge the plan for the force-and-acceleration investigation
The claim: this plan is valid, reproducible, safe and heritage-respecting. Decide which actions support it.
Claim: This plan is a valid, reproducible, safe and heritage-respecting investigation of how applied force affects the acceleration of a trolley.
Hold the moving mass, ramp angle and gate spacing identical so only the applied force changes between runs.
Time five runs at each force and average them, using one fixed gate-triggering rule to cut reaction-time scatter.
If a grooved rock or arrangement of stones is found at the outdoor site, leave it undisturbed and seek guidance from Traditional Owners before going further.
Quietly add masses to the trolley between runs and let the ramp angle drift to get a livelier set of results.
Skip clamping the ramp and let the trolley run off the bench onto the floor to save resetting time.
Decide whether each statement is evidence for the claim, or not.
Why this matters
A result others can trust has to be both valid and reproducible: it measures the right quantity, with confounds and sources of error controlled, and it is written so anyone can repeat it and get consistent numbers. For real investigations that also means a worked risk assessment and acting ethically and respectfully on Country and Place. Engineers, physicists and heritage officers plan exactly this way, so that the evidence about how force and motion behave is reliable and the land and its cultural heritage are protected.
Quick self-check
1. You roll a loaded trolley down a ramp and time it past two gates to find its acceleration, so you can test whether a doubled pushing force doubles the acceleration. What makes the investigation valid?
2. A reproducible motion investigation is one where...
3. Your timing gate is started and stopped by hand, and the times jump around from run to run. This is best described as a...
4. Which belongs in the risk assessment for a trolley-and-ramp investigation?
5. While looking for an outdoor slope to run the ramp investigation, you find a rock with old grinding grooves and an arrangement of stones. The right action is to...