6.5 Scientific Method, Measurement Systems, and Laboratory Instruments

6.5 Scientific Method, Measurement Systems, and Laboratory Instruments

Core Principle: General Science does not only ask what — it asks how we know. A predictable share of the 15 scored GS items test the scientific process, the units that science is written in, and the instrument used to measure a given quantity. At roughly 48 seconds per item these are among the fastest points on the subtest, because none of them require calculation.

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1. The Scientific Method

The method is a cycle, not a straight line: results feed back into new questions.

   OBSERVE  ->  QUESTION  ->  HYPOTHESIS  ->  EXPERIMENT  ->  ANALYSE  ->  CONCLUDE
      ^                                                                        |
      +------------------------ REPLICATE & REFINE ----------------------------+

The vocabulary the exam tests

Designing a controlled experiment

Change one variable at a time. If two variables change together, no conclusion can be attributed to either. A placebo is an inert treatment given to a control group; a double-blind design conceals group assignment from both subject and experimenter to prevent bias.

Correlation is not causation. Two quantities can move together because one causes the other, because both are driven by a third factor, or by coincidence. Only a controlled experiment distinguishes among those.

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2. The SI System

Science is written in the International System of Units (SI), built on seven base units.

Everything else is derived: the newton (kg·m/s²) for force, the joule (N·m) for energy, the watt (J/s) for power, the pascal (N/m²) for pressure, the hertz (1/s) for frequency, the coulomb (A·s) for charge, and the volt (W/A) for potential difference.

Metric prefixes

Because every step is a power of ten, metric conversion is decimal-point movement: 2.4 km = 2,400 m = 240,000 cm. Contrast that with the imperial system, where 1 mile = 5,280 ft and 1 gallon = 128 fluid ounces — arbitrary factors that must be memorised.

Mass versus weight. Mass is the quantity of matter, measured in kilograms with a balance, and it is unchanged by location. Weight is the force gravity exerts on that mass, measured in newtons with a spring scale, and it changes with local gravity. An astronaut's mass is identical on the Moon; their weight is about one sixth. This distinction is tested in both General Science and Mechanical Comprehension.

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3. Temperature Scales

$^\circ F = \frac{9}{5}(^\circ C) + 32 \qquad ^\circ C = \frac{5}{9}(^\circ F - 32) \qquad K = ^\circ C + 273$

Kelvin is an absolute scale with no negative values, and its degree is the same size as a Celsius degree — which is why a change of 10 °C is a change of 10 K. Fahrenheit and Celsius read the same value at −40°, a fact that shows up as a distractor check.

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4. Accuracy, Precision, and Significant Figures

• Accuracy is closeness to the true value.
• Precision is repeatability — how tightly clustered repeated measurements are.

A rifle that groups three rounds inside a coin but two feet left of the bullseye is precise but not accurate; the sight is offset by a fixed amount, which is systematic error. Rounds scattered evenly around the bullseye are accurate but not precise, reflecting random error. Systematic error is corrected by calibration; random error is reduced by averaging repeated measurements.

Significant figures communicate how precise a measurement is. A length recorded as 3.0 cm claims precision to a tenth of a centimetre; 3.00 cm claims precision to a hundredth. A calculated result should never be reported with more significant figures than the least precise measurement that fed it.

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5. Matching the Instrument to the Quantity

Reading a meniscus: water in a glass cylinder curves upward at the edges, so the true volume is read at the bottom of the curve, with your eye level with that point. Reading from above or below introduces parallax error, a term the exam uses by name.