Types of Measurement Tools

Measurement Connects Values and Units

Every time we weigh food ingredients, look at a speedometer, read body temperature, or measure the length of a table, we are measuring something. In physics, measurement is the experimental process of obtaining one or more quantity values. A measurement unit supplies the shared reference.

A quantity value is the numerical value multiplied by the unit.

For example, if the length of an object reads $8 \text{ cm}$, then $8$ is the numerical value and $\text{cm}$ is the unit.

Choose a tool that measures the required quantity and covers the expected range with suitable resolution. An unsuitable tool gives an unusable result even when its scale is read correctly.

Comparing Measurement Tools

Use the following interactive model to compare three measurement tools. Choose a tool, rotate the view, then check the measured quantity, unit, and scale reading.

Choosing a Measurement Tool

Identify the quantity before measuring. Its expected range and required resolution determine the measuring tool, while the quantity determines the unit used to report the result.

SI is the international abbreviation for Système international d’unités, the International System of Units. In practice, SI provides shared reference units, so measurements from different laboratories and countries can be compared directly.

In SI, base quantities that often appear at the start of physics include length, mass, and time. Their base units are $\text{m}$ for length, $\text{kg}$ for mass, and $\text{s}$ for time.

Reading Measurement Results

The three examples below practice reading a measuring tool from its starting point, displayed value, and unit. Each example also explains what the number means.

Length

When reading a ruler, check both ends of the object. If the object starts at $0 \text{ cm}$ and ends at $8 \text{ cm}$, the result is:

If the object starts at $1 \text{ cm}$ and ends at $7 \text{ cm}$, its length is not $7 \text{ cm}$.

Mass

Mass describes how strongly an object resists a change in motion. A balance measures it by comparison with known masses. If the balance reads $250 \text{ g}$, then the object's mass is:

In the lab above, the balance is modeled as an equal-arm balance. The object sits on the left pan, while a $250 \text{ g}$ reference mass sits on the right pan. If the object mass is greater than $250 \text{ g}$, the object side moves down. If it is smaller than $250 \text{ g}$, the reference side moves down. If both masses are equal, the balance is in equilibrium.

For full SI writing, the mass can be converted to kilograms.

Time

A stopwatch is used when we want to measure the duration of an event. If the stopwatch reads $12.8 \text{ s}$, the event lasts for $12.8$ seconds.

Measurement Tools Around Us

Every physical quantity has tools that read it directly. The table pairs a common instrument with the quantity it reports, so you can choose the tool from the quantity you need.

Choosing the instrument is the first decision and reading its unit is the second. A thermometer reports temperature in degrees Celsius, while a balance reports mass in kilograms, so the number alone does not say what was measured.

Finding a Length from Two Readings

A pencil is placed on a ruler. The left end of the pencil is at $2 \text{ cm}$, while the right end is at $15 \text{ cm}$. The pencil length is:

So, the pencil length is $13 \text{ cm}$.

The pencil example contains three parts: length is the target quantity, the ruler is the instrument, and $13\text{ cm}$ is the result with its unit. Naming all three lets another reader identify what was measured and compare it with lengths reported in the same unit.

The definitions of measurement and quantity value follow separate entries in the International Vocabulary of Metrology.