Taking a measurement is one skill, and judging what it shows is another. This section trains the second: what spread means, how to draw a line, how to spot a systematic error, and how precisely to state a result.
It sits inside SPM Physics measurement. It is most useful after you can read instruments and plot graphs.
Which four skills does it cover?
- Distinguishing resolution from repeated-measurement spread
- Explaining why a graph line should not join every dot
- Identifying a systematic offset from a supplied calibration example
- Stating a conclusion at a precision justified by the data
What does one experiment show?
An original experiment: a student times 20 swings of a pendulum four times and gets 28.4 s, 28.9 s, 28.1 s and 28.6 s. The stopwatch shows hundredths of a second.
Resolution of 0.01 s looks precise, but the four readings differ by up to 0.8 s. That spread, about 0.4 s either side of the mean, is the real uncertainty. A line on a graph of period against length should pass through the scatter and not join every dot.
If the stopwatch starts late every time, all the readings shift in the same direction, which is a systematic offset and repeating does not remove it. Finally, the conclusion should not claim more digits than the data justify.
Who should start where?
- You quote the instrument resolution as your uncertainty: start with resolution and spread.
- You draw zigzag lines: start with the best-fit line.
- Your graph does not pass through the origin when it should: start with systematic offsets.
- You copy every digit from a calculator: start with precision.
Practise with the section practice set.
Need help with data questions?
A teacher can give you a fresh data set and listen as you state the result and its limit. If you would like that, see online one-to-one Physics tuition.