Unit 00 · Day 3
Measurement & SI Units
Use suitable units and tools, record repeated measurements honestly, and establish a complete science-notebook standard.
Learning objectives
- Select appropriate units and measuring tools
- Record repeated measurements with units
- Describe variation without changing raw data
Materials
- Metric ruler or measuring tape
- Three safe household objects
- Science notebook
- Calculator optional
Follow the written procedure. Stop whenever materials, supervision, health needs, or environmental conditions differ from the lesson.
1. Quantity, unit, and tool
A measurement needs a number and a unit. Select a tool whose range and smallest marked interval are appropriate for the object or event.
| Quantity | Common SI unit | Possible tool |
|---|---|---|
| Length | meter (m) | ruler or measuring tape |
| Mass | kilogram (kg) or gram (g) | balance or scale |
| Time | second (s) | stopwatch |
| Temperature | kelvin (K); °C often used in school laboratories | thermometer |
| Volume | cubic meter (m³); liter and milliliter commonly used | graduated container |
2. Repeated-measurement activity
- Select three safe household objects.
- Measure one dimension of each object three times in millimeters.
- Record every result without changing an inconvenient value.
- Calculate the range: largest measurement minus smallest measurement.
- Explain one likely source of variation.
| Object | Trial 1 | Trial 2 | Trial 3 | Range |
|---|---|---|---|---|
3. Notebook standard
Every investigation record should include the date, question, materials, safety tier, method, raw data, calculations, observations, changes to the method, conclusion, limitations, and sources.
Create the first dated notebook entry and attach or copy your measurement table.
4. Essential question & measurement model
Essential question: What makes a measurement scientifically useful rather than merely precise-looking?
A useful measurement connects a quantity, an appropriate tool, a stated unit, and honest reporting of variation. No measurement is perfectly exact. The markings on a ruler, the response time of a stopwatch, the calibration of an instrument, the way an object is positioned, and ordinary human judgment can all affect the recorded value.
Resolution is the smallest change an instrument can meaningfully display. Repeatability asks whether repeated measurements under similar conditions are close to one another. Accuracy concerns how close a measurement is to an accepted or well-supported reference value when such a reference is available. These ideas are related but not interchangeable.
5. Worked measurement example
A learner measures the same card three times and records 124 mm, 125 mm, and 124 mm.
- Raw data: keep all three values exactly as observed.
- Range: 125 mm − 124 mm = 1 mm.
- Mean: (124 + 125 + 124) ÷ 3 = 124.3 mm, rounded to a level appropriate for the measuring method.
- Interpretation: the small range suggests good repeatability, but repeatability alone does not prove accuracy.
- Possible source of variation: the ruler may have been aligned slightly differently on each trial.
If a digital display shows 124.3000 mm, does the extra number of decimal places automatically make the measurement more accurate? Explain what additional information you would need before trusting that precision.
6. Vocabulary & misconception checks
- Quantity: the property being measured, such as length, mass, time, or temperature.
- Unit: the agreed scale used to report the quantity.
- Resolution: the smallest change an instrument can meaningfully distinguish.
- Repeatability: closeness of repeated measurements made under similar conditions.
- Accuracy: closeness to an accepted or well-supported reference value.
- Uncertainty: the recognized limitation in how precisely a value is known.
Correct the statement: “If I measure something three times and get the same answer every time, the answer must be accurate.” What could make repeated measurements consistently wrong?
7. Retrieval, mentor measurement review & portfolio evidence
- Why must every measurement include a unit?
- What does the range of repeated measurements tell you?
- How is resolution different from accuracy?
- Why should an inconvenient measurement remain in the raw-data record?
Show Archaemenes one repeated-measurement set and your explanation of its variation. Ask for one question that would help distinguish tool limitation from procedure variation. The mentor supports reasoning; it does not alter or replace your raw data.
Keep your repeated-measurement table, range calculation, one source-of-variation explanation, and one statement about the measuring tool's resolution.