Improving Experiments

Practical Skills - OCR A-Level Physics

Timing Multiple Oscillations

When timing a single oscillation, human reaction timeThe delay between observing an event and pressing the button, typically around 0.2 s. This dominates the uncertainty when timing short events. ($\sim 0.2$ s) introduces significant uncertainty.
By timing 10 or more complete oscillations and dividing, the percentage uncertainty due to reaction time is reduced by a factor equal to the number of oscillations.
Example: timing 1 oscillation (2.0 s) gives $\pm 10\%$ uncertainty. Timing 10 oscillations (20.0 s) gives only $\pm 1\%$ uncertainty.
This technique works for any periodic motion: pendulums, springs, rotating systems.

Fiducial Markers

Key Definition

Fiducial Marker
A fixed reference point used to improve the accuracy of timing measurements. Placed at the centre of oscillation (the equilibrium position) where the object moves fastest.

Start and stop timing as the object passes the marker, sighting it at its highest speed (the equilibrium position) for the most precise moment.
A pin with coloured tape (e.g. blu-tack) placed at the centre of oscillation provides a clear reference.
Position yourself at eye level with the marker to reduce parallax error.
Always sight the object passing the marker in the same direction for consistency.

Set Squares & Plumb Lines

A set squareA triangular tool used to check whether two objects are at right angles (90 degrees) to each other, or whether surfaces are parallel. checks that objects are at right angles, vertical, or parallel.
A plumb lineA weight suspended on a string that naturally hangs vertically due to gravity, used to verify that a setup is truly vertical. (a weight on a string) naturally hangs vertically, used to verify vertical alignment of apparatus.
Even small misalignments can introduce systematic errors affecting all measurements equally.
Example: a ruler tilted by just 5 degrees introduces a systematic error of approximately 0.4%, which for a 1.00 m pendulum means an error of about 4 mm.
These errors cannot be reduced by repeating measurements -- the setup itself must be corrected.

Common Experimental Limitations

Parallax errorError caused by reading a scale from an angle rather than perpendicular to it. Reduced by positioning the eye directly in line with the scale. when reading scales -- always read perpendicular to the scale.
Not repeating measurements to reduce random errors.
Not checking for zero errors before starting.
Using equipment with poor precision or resolution (e.g. a ruler instead of a micrometer for small measurements).
Difficult-to-control variables such as room temperature.
Unwanted heating effects in electrical circuits causing resistance to change.

Writing Evaluations

Use precise scientific language, not vague statements. Instead of "the time wasn't measured accurately", write: "Using a stopwatch introduced an uncertainty of $\pm 0.2$ s due to reaction time."
Calculate percentage errors to quantify the impact of each limitation.
Suggest specific, practical improvements for each limitation identified.
Identify and explain any anomalous results.
State whether the experiment supports or contradicts the hypothesis, and with what level of confidence.

Common MistakeMEDIUM

Wrong: Writing vague evaluation statements like "We could have been more careful" or "Results were not very accurate."
Right: Be specific: "The main source of uncertainty was reaction time ($\pm 0.2$ s on a 2.0 s measurement = 10%). This could be reduced by timing 10 oscillations."