Launch practical
About 60 minutes
- Set up the bench with the 13 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 20 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. The logger works out each speed from the card’s length and how long it blocks the beam, then a = (v₂ − v₁) ÷ t.
What it shows
F = m a
A resultant force makes a mass accelerate: resultant force = mass × acceleration. Double the force on the same mass and the acceleration doubles; double the mass with the same force and it halves. The hanging weight provides the pull; the resultant force also accounts for friction and the runway slope. The mass is everything the string accelerates: the trolley, the hanger and every mass on either.
The graph to expect. With friction compensated and the total mass constant, acceleration is directly proportional to force: a straight line through the origin. Friction left in shifts the line so it crosses the force axis at the friction force.
On the AQA 2027 Physics Equations Sheet
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Dynamics trolley | The object being accelerated. | — |
| 10 cm card on the trolley | Cuts the light gate beams. | — |
| Runway (1.3 m) | A smooth, straight track for the trolley. | — |
| Wooden block | Tilts the runway to compensate for friction. | — |
| Bench pulley and clamp | Turns the hanging weight into a horizontal pull. | — |
| String | Joins the trolley to the mass hanger. | — |
| 20 g mass hanger and four 20 g slotted masses | Their weight is the accelerating force (0.2–1.0 N). | — |
| Four 200 g trolley masses | Change the mass of the trolley in part 2. | — |
| Two light gates and a data logger | Measure the speed at each gate and the acceleration. | 0.01 m/s² |
| Stopwatch | The other way to time the trolley. | 0.01 s |
| Metre rule | Measures the card and the distances along the runway. | 1 mm |
| Digital balance | Measures the mass of the trolley. | 0.1 g |
| Box of sand and an end stop | Catch the falling masses and the trolley. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical differently. Your course setting picks one; you can change it before you start.
Why: Each version is a fair way to do the practical. The bench, the readings and the exam questions follow your board.
Measure the card
Measure the card with the metre rule and type its real length into the data logger.
Why: The logger works out each speed as card length ÷ the time the card blocks the beam. A wrong length makes every speed, and every acceleration, wrong by the same proportion.
Clamp the pulley and hang the masses
Clamp the bench pulley to the end of the bench. Tie the string to the trolley, pass it over the pulley and hang the mass hanger on the end. Line the pulley up so the string is horizontal and in line with the runway.
Why: A string at an angle pulls partly up or sideways, so less of the weight pulls the trolley along. A string rubbing on the bench or the side of the pulley adds friction.
Compensate for friction
Unhook the string. Put the wooden block under the start end of the runway and give the trolley a gentle push. Move the block until the trolley rolls at a steady speed all the way down.
Why: On the slope a small part of the trolley’s weight acts down the runway. It balances the friction, so the resultant force on the trolley is just the pull of the string.
Choose how to measure the acceleration
Choose how you will measure the acceleration of the trolley.
Do: Light gates and a data logger.
Why: You need the trolley’s speed at two places and the time between them, or a time over a measured distance from rest.
Place the second light gate
Choose where to clamp the second light gate. Gate 1 is 20 cm from the start line. The hanger falls about 70 cm before it lands in the sand.
Do: 55 cm from the start.
Why: The string only pulls while the hanger is falling. Once it lands, the trolley stops accelerating.
Load the trolley
Choose where the 20 g slotted masses for part 1 (changing the force) come from: start with all four on the trolley and move one to the hanger each time, or add new ones to the hanger from the box.
Do: Move masses from trolley to hanger.
Why: The string accelerates the trolley and the hanging masses together, so the mass being accelerated is the trolley, the hanger and every mass on either of them.
Sand box and end stopSafety
Put a box of sand on the floor under the hanger, and an end stop at the end of the runway.
Why: Falling masses can land on feet, and a trolley running off the end can fall off the bench.
Take the readings
Part 1: 20, 40, 60, 80 and 100 g on the hanger, each mass moved from the trolley to the hanger so the total mass stays constant. Part 2: 100 g on the hanger, 0 to 800 g added to the trolley. Each time, release the trolley from the same line and record the acceleration from the light gates (or the time between two lines); two trials each, a third if they differ. Plot acceleration against force for part 1, and against 1 ÷ mass for part 2.
Repeats: Do each run twice. If the two values are not similar, repeat again (AQA’s method), then take the mean. A mean reduces the effect of random errors (reaction time, the trolley’s wheels) but not systematic ones (friction, a wrong card length, a gate after the hanger lands).
Make safe and clear upSafety
Lift the masses off the hanger and the trolley and put them back in the box. Switch off the data logger and unplug the light gates. Untie the string, unclamp the pulley and put the box of sand away. Take the block from under the runway and put the trolley away.
While you take readings
Release from rest
Hold the trolley still with its back on the start line, then let go without pushing it. Do the same every run.
Why: The method assumes the trolley starts from rest at the same place each time.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Masses falling from the hanger when it reaches the floor | Bruised or broken toes | Put a box of sand under the hanger and keep feet clear of it. |
| Trolley running off the end of the runway | The trolley falls off the bench onto someone’s feet | Put an end stop at the end of the runway and use small forces only. |
| Light gate leads trailing across the bench and floor | Someone trips, or pulls the logger off the bench | Run the leads along the back of the bench, away from where people walk. |
| String snapping under load | The masses drop and the trolley lurches into someone | Check the string is not frayed and tie the knots firmly before you start. |
Variables
- Independent: you change
- Accelerating force (weight of the hanging masses: the resultant force on the trolley, hanger and masses together) in N
- Dependent: you measure
- Acceleration of the trolley in m/s²
- Control: you keep the same
- Total mass of the trolley, hanger and masses in kg. Move masses from the trolley to the hanger rather than adding new ones.
- Slope of the runway. Tilt once to compensate for friction, then leave it.
- Positions of the light gates and the start line. Clamp the gates and release from the same line every run.
Results from a specimen run
| Mass of the trolley in g |
|---|
| 690.7 |
| Mass on the hanger in g | Force in N | Total moving mass in kg | Trial 1 in m/s² | Trial 2 in m/s² | Trial 3 in m/s² | Mean acceleration in m/s² |
|---|---|---|---|---|---|---|
| 20 | 0.20 | 0.791 | 0.25 | 0.24 | — | 0.25 |
| 40 | 0.39 | 0.791 | 0.50 | 0.50 | — | 0.50 |
| 60 | 0.59 | 0.791 | 0.73 | 0.74 | — | 0.74 |
| 80 | 0.78 | 0.791 | 1.01 | 0.98 | 0.99 | |
| 100 | 0.98 | 0.791 | 1.26 | 1.24 | — | 1.25 |
| Mass added to the trolley in g | Total mass in kg | 1 ÷ total mass in 1/kg | Trial 1 in m/s² | Trial 2 in m/s² | Trial 3 in m/s² | Mean acceleration in m/s² |
|---|---|---|---|---|---|---|
| 0 | 0.791 | 1.265 | 1.26 | 1.24 | — | 1.25 |
| 200 | 0.991 | 1.009 | 1.01 | 0.97 | — | 0.99 |
| 400 | 1.191 | 0.840 | 0.82 | 0.83 | — | 0.83 |
| 600 | 1.391 | 0.719 | 0.71 | 0.71 | — | 0.71 |
| 800 | 1.591 | 0.629 | 0.62 | 0.61 | — | 0.62 |
Anomaly spotted: Acceleration, 80 g on the hanger, trial 1, retaken.
Processing the data
- The force with 60 g on the hanger
- 0.59 N
- The acceleration with 100 g on the hanger
- 1.26 m/s²
- Gradient (acceleration per newton)
- 1.28 1/kg
- Mass estimate (1 ÷ gradient, constant-mass model)
- 0.783 kg
- Where does your line cross?
- It goes through the origin (within the scatter): acceleration is directly proportional to force, so friction was compensated.
- Where did the pull go?
- 3 of 3 right first time
- 1 ÷ mass
- 0.840 1/kg
- Shape of the graph
- A straight line: acceleration is inversely proportional to the mass (a ∝ 1 ÷ m).
- Gradient (the accelerating force)
- 0.987 N
| Quantity | Specimen run | Accepted |
|---|---|---|
| Mass being accelerated (1 ÷ gradient of a against F) | 0.783 kg | 0.791 kg |
| Accelerating force (gradient of a against 1 ÷ m) | 0.987 N | 0.980 N |
Across 120 simulated careful groups, mass being accelerated (m) came out between 0.621 and 1.09 kg (median 0.858). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Leave it at 10 cm
Hand-cut cards are 9.7–10.3 cm long. If yours is not exactly 10 cm, every acceleration is too big or too small by the same percentage (up to 3 %), so the gradient and the mass from it are out too (a systematic error).
The fix: Measure the card. The logger works out each speed as card length ÷ the time the card blocks the beam. A wrong length makes every speed, and every acceleration, wrong by the same proportion.
Method: Leave the runway flat
Friction (about 0.04–0.15 N) acts against the trolley in every run, so every acceleration is less than F ÷ m. Your line of acceleration against force is still straight, but it crosses the force axis at about the friction force instead of going through the origin (a systematic error).
The fix: Compensate for friction. On the slope a small part of the trolley’s weight acts down the runway. It balances the friction, so the resultant force on the trolley is just the pull of the string.
Method: Stopwatch over a measured distance
You time the trolley from release to a chalk line, then a = 2 s ÷ t². Your reaction time (about 0.2 s, different every time) is a big fraction of a 1–2 s run, so each acceleration can be 10 % out and repeats spread much more (random error).
The fix: Choose how to measure the acceleration. You need the trolley’s speed at two places and the time between them, or a time over a measured distance from rest.
Acceleration, 20 g on the hanger, trial 1: Typed 0.525 m/s²
The fix: That is the speed at gate 2, in m/s. The acceleration is the bottom line of the logger’s screen, in m/s².
Acceleration, 20 g on the hanger, trial 1: Typed 0.315 m/s²
The fix: That is the speed at gate 1, in m/s. The acceleration is the bottom line of the logger’s screen, in m/s².
Exam answer: Leaving out: resultant force = mass × acceleration, F = m a (or any correct rearrangement)
The fix: Resultant force = mass × acceleration, F = m a. Find it on the full AQA 2027 Physics Equations Sheet, then apply it.
Exam answer: Leaving out: A component of the trolley’s weight acts down (parallel to) the runway
The fix: On a slope, part (a component) of the trolley’s weight acts down the runway.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Newton’s second law · Knowledge[1 mark]
Write down the equation that links acceleration (a), mass (m) and resultant force (F).
Worked answer and how the marks are given
A full-mark answer
resultant force = mass × acceleration
F = m aMark scheme
- resultant force = mass × acceleration, F = m a (or any correct rearrangement)
Where marks are usually lost
- Missing: resultant force = mass × acceleration, F = m a (or any correct rearrangement). Resultant force = mass × acceleration, F = m a. Find it on the full AQA 2027 Physics Equations Sheet, then apply it.
- Question 2Uncertainty from repeats · Applying knowledge[2 marks]
Table 2 shows your repeat readings with 60 g on the hanger.
Table 2 Trial Acceleration in m/s² 1 0.73 2 0.74 Calculate the uncertainty in the mean acceleration.
Uncertainty = ± m/s²
Worked answer and how the marks are given
A full-mark answer
Range = 0.74 − 0.73 = 0.01 m/s²
Uncertainty = ±0.01 ÷ 2 = ±0.005 m/s²Mark scheme
- Range = 0.74 − 0.73 = 0.01 m/s² (method)
- Uncertainty = ±0.005 m/s² (half the range)
- Allow answers from 0.004800 to 0.005200 m/s² (the marker's tolerance; correct rounding of the student's own working is also accepted).
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 0.01 m/s² (gains 1 mark). 0.01 m/s² is the range. The uncertainty in the mean is ± half the range: ±0.005 m/s².
- Missing: Range = 0.74 − 0.73 = 0.01 m/s². Find the range: largest value − smallest value.
- Missing: Uncertainty = ±0.005 m/s² (half the range). The uncertainty is ± half the range.
- Question 3Describing the method · Knowledge[6 marks]
A student wants to investigate how the acceleration of a trolley depends on the resultant force on it.
The student has a trolley, a runway, a bench pulley, string, a mass hanger, slotted masses, two light gates and a data logger.
Describe a method the student could use. Your answer should include how the mass is kept constant.
Worked answer and how the marks are given
A full-mark answer
1. Put the trolley on a runway with a pulley clamped at the end of the bench. Tie a string to the trolley, pass it over the pulley and attach a mass hanger.
2. Tilt the runway with a block until the trolley rolls down at a steady speed when pushed, to compensate for friction.
3. Clamp two light gates over the runway, connected to a data logger, so the card on the trolley passes through both before the hanger hits the floor.
4. Put all the slotted masses on the trolley. Release the trolley from rest at the same starting line and record the acceleration from the data logger.
5. Move one mass from the trolley to the hanger to increase the force, keeping the total mass constant. The force is the weight of the hanging masses, W = m g.
6. Repeat each force and calculate a mean acceleration.
7. Plot a graph of acceleration against force.Mark scheme
Indicative content:
- Trolley on a runway, string over a pulley to a mass hanger
- Tilt (raise one end of) the runway so the trolley rolls at a steady speed, to compensate for friction
- Measure the acceleration with light gates and a data logger (or time over a measured distance)
- Change the force by changing the masses on the hanger (force = weight, W = m g)
- Keep the total mass constant: move masses from the trolley to the hanger
- Release the trolley from rest from the same starting position each time
- Repeat for each force and calculate a mean
- Plot a graph of acceleration against force
- Place the second gate so the trolley passes it before the hanger hits the floor
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Friction not compensated (runway flat or not tilted enough) | Systematic | Every acceleration is lower than F ÷ m, so the line crosses the force axis at the friction force instead of the origin. | Tilt the runway until a pushed trolley rolls at a steady speed (equal times through both gates). |
| Total mass not kept constant (new masses added to the hanger from the box) | Systematic | The mass being accelerated grows with the force, so the bigger forces give less acceleration than expected and the graph curves. | Start with the slotted masses on the trolley and move them to the hanger one at a time. |
| Reaction time starting and stopping a stopwatch | Random | Each time is out by a different amount (about 0.1 s), so the accelerations scatter by about 10 %. | Use light gates and a data logger instead of a stopwatch. |
| Second light gate (or finish line) after the point where the hanger lands | Systematic | The trolley stops accelerating before it is timed at the end, so every acceleration is too low. | Put the second gate where the trolley passes it before the hanger hits the floor. |
| Card length typed into the logger not measured | Systematic | Every speed, and every acceleration, is wrong by the same percentage. | Measure the card with a ruler and type the real length into the logger. |
| Small differences from run to run (wheels, string, how the trolley is let go) | Random | Repeats differ by a few hundredths of a m/s². | Repeat each run and calculate a mean, leaving out anomalies. |
Repeatable. When you repeat a run with the same trolley, force and light gates, your accelerations agree closely.
Reproducible. Another group, with a different trolley or a stopwatch instead of light gates, also finds that acceleration is proportional to the resultant force.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 7 | This page | — |
| Edexcel | 2.19 | Similar, with differences | Masses are moved between trolley and hanger so total mass stays constant, on a ramp with light gates or a datalogger. |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.