Measurement, Units and Scientific Method
Quantities, units and prefixes
Every physical quantity is a number together with a unit. BGCSE candidates must handle the SI base units (metre, kilogram, second, ampere, kelvin, mole, candela) and the common derived units (newton, joule, watt, pascal, coulomb, volt, ohm). Be fluent converting between prefixes: kilo (10³), centi (10⁻²), milli (10⁻³), micro (10⁻⁶). A frequent trap is mixing prefixes in one calculation, for example dividing millimetres by seconds and reporting metres per second without converting first. Convert everything to SI before substituting.
Instruments and precision
Know the resolution of each instrument: metre rule (1 mm), vernier caliper (0.1 mm or 0.05 mm), micrometer screw gauge (0.01 mm), measuring cylinder (typically 1 cm³), thermometer (often 1 °C), stopwatch (0.01 s). Resolution limits precision, not accuracy. Repeat readings and average them; for a timing, measure many oscillations and divide. Avoid parallax error by reading at eye level, and check for zero error on calipers, micrometers and balances.
Scientific method
A sound investigation states a hypothesis, identifies independent, dependent and controlled variables, and records data in a table with units in the headings. Graphs should have labelled axes with units, a sensible scale and plotted points; a line of best fit reveals the relationship. Identify anomalies and explain, rather than silently delete, them. Conclusions should refer back to the hypothesis, and evaluations should suggest specific improvements (more repeats, finer instrument, better control of variables).
Worked rule of thumb
When asked for a percentage uncertainty, use (uncertainty ÷ measured value) × 100%. If a stopwatch reading is 2.00 s with a reaction uncertainty of 0.2 s, the percentage uncertainty is (0.2/2.00)×100% = 10%. Timing 20 swings and dividing by 20 reduces the reaction uncertainty per swing to 0.01 s, a clear improvement worth stating in an evaluation.