By Cyril KimbiThe physics practical is the paper where preparation is most unevenly distributed. Candidates revise theory for months and walk into the practical having done a handful of experiments, then lose marks on things that have nothing to do with physics — table headings, decimal places, the line of best fit.
Where the marks actually are
| Component | What earns the mark | The usual loss |
|---|---|---|
| Table | Quantity and unit in each heading; consistent decimal places | Units written beside each value instead of in the heading |
| Readings | Sensible range, repeated where appropriate | Too few readings, or clustered together |
| Graph | Sensible scale, labelled axes with units, points plotted accurately | Scale chosen so the points occupy a corner |
| Line of best fit | A single straight line with points balanced either side | Dot-to-dot, or a line forced through the origin |
| Gradient | A large triangle, read from points on the line | Using two data points instead |
| Conclusion | Answers the question, with the unit | Stating a number without saying what it means |
Notice how little of this is physics. That is the point, and it is good news: this is all learnable technique, and it is where the reliable marks are.
The table
Set it out before you take a single reading. Deciding the columns first prevents the scramble that produces messy tables and lost marks.
- Units go in the heading, once — written as length / m or t / s, not repeated beside every value.
- Consistent decimal places down each column, matching the precision of the instrument. If the ruler reads to a millimetre, every entry has the same number of decimal places.
- Include the processed column the question asks for, with its own heading and unit.
- Never overwrite a reading. Cross it out and rewrite beside it.
Significant figures are the most common quiet loss. Your processed values should not carry more precision than the raw readings that produced them.
The graph
Choose the scale so the plotted points fill at least half of each axis. A graph squeezed into one corner loses a mark before anything is plotted, and it also makes an accurate gradient impossible.
Use scales you can read — 1, 2, 5 or 10 units per square. A scale of 3 units per square costs you accuracy and time on every point.
Label both axes with quantity and unit. Plot points as small crosses rather than dots, which are hard to see under the line.
Line of best fit, and the gradient
Two errors account for a large share of graph marks lost.
Joining the points. The line of best fit is a single straight line with roughly equal numbers of points either side. It is not a dot-to-dot, and it does not have to pass through the origin unless the physics requires it.
Taking the gradient from data points. The gradient must be read from two points on the line, chosen far apart — ideally spanning most of the graph. Using two plotted data points defeats the purpose of drawing a best fit at all, and a small triangle magnifies your reading error.
Mark the triangle on the graph and show the substitution in your working. Method marks are frequently awarded even when the final value is out.
Uncertainty and sources of error
When asked for sources of error, avoid the generic answers every candidate writes. “Human error” and “the equipment was faulty” score nothing.
What scores is specific and relevant to this experiment: parallax in reading a scale at eye level, reaction time when starting and stopping a stopwatch by hand, heat lost to the surroundings, difficulty judging the exact moment of an event.
And when asked for an improvement, propose something that addresses the error you actually named. “Be more careful” is not an improvement; “use a light gate to remove reaction time” is.
Preparing without a laboratory
This is the real constraint for many candidates, and most of the marks are reachable anyway — because most of them are in the recording and processing, not in the apparatus.
- Take the data tables from past papers and do everything after the readings: process the columns, plot the graph, draw the line, find the gradient, write the conclusion.
- Draw twenty graphs. Scale, axes, plotting and best fit are pure technique and improve fast with repetition.
- Practise the write-up — method, sources of error, improvements — in prose, from memory.
- Do the simple experiments that need no laboratory: a pendulum with string and a phone timer, a ruler and a spring, timing a rolling object down a slope.
The physics past papers supply the data and the schemes, and the offline study guide covers building a local library of them in one connected session.
On the day
Read the whole paper before touching the apparatus, so you know what the readings will be used for. Take readings across the full range available rather than clustering them. Record raw readings immediately, exactly as the instrument gives them — process later, never in your head. And leave time for the graph: it carries a large share of the marks and cannot be rushed.
Frequently asked questions
Do I need a laboratory to prepare?
Not for most of the marks. Recording, processing, graph work and the write-up are all practisable on paper with past-paper data.
Where should units go in a table?
In the column heading, once, written as quantity / unit. Not beside each value.
Should the line of best fit pass through the origin?
Only if the physics requires it. Otherwise draw the line the points support.
How do I calculate the gradient correctly?
From two widely separated points on the line, not from data points, showing the triangle and the substitution.
What counts as a valid source of error?
Something specific to this experiment — parallax, reaction time, heat loss. “Human error” scores nothing.
