Practical Skills and Experimental Techniques
Measurement and units
SEC Physics practical questions test apparatus, SI units, prefixes, significant figures, uncertainty, variables and graph work.
- Base SI units include metre, kilogram, second, ampere, kelvin and mole; prefixes include milli (10⁻³), centi (10⁻²), kilo (10³) and mega (10⁶).
- Read scales to the smallest division plus one estimated digit where appropriate; vernier calipers typically read to 0.01 cm (0.1 mm), micrometer screw gauges to 0.01 mm.
- Repeat readings and average to reduce random error; check for zero error and parallax.
- Independent variable on the x-axis, dependent variable on the y-axis; label axes with quantity and unit; draw a line of best fit, not point-to-point joins.
- Accuracy is closeness to the true value; precision is consistency of repeated measurements.
Worked example
A pendulum of length 0.80 m is timed for 20 complete swings; the total time is 36.0 s. Find the period and estimate g using T = 2π√(L/g).
T = 36.0/20 = 1.80 s.
Rearrange: g = 4π²L/T² = 4 × 9.87 × 0.80 / 1.80² = 31.6/3.24 ≈ 9.75 m/s².
Reasoning: timing many swings reduces the effect of human reaction time; divide the total time by the number of swings.
Traps
Forgetting to divide total time by the number of swings; mixing cm and m; quoting more significant figures than the apparatus justifies; joining plotted points dot-to-dot instead of drawing a best-fit line; ignoring a zero error.