1.4
Units and Standards of Measurement
A physical quantity is defined either by specifying its measurement method or by stating how it is calculated from other measurements. For example, co…
A physical quantity is any number that describes a physical phenomenon quantitatively. For instance, length is a physical quantity that describes the distance between two points.
The measurements of physical quantities are expressed in terms of units. The mass of your body can be expressed in kilograms and your height in terms of meters.
Physical quantities expressed without standard units are difficult to understand. For example, if the height of the Eiffel Tower is reported to be 300, without providing units the context of the number is unclear. This means the Eiffel tower could be relatively tall, or very short.
In order to measure and report the various properties of matter accurately, a common system of measurement has been developed known as the International System of Units or the SI units.
The SI system recommends using seven base quantities and their units, which form the International System of Quantities.
View the full transcript and gain access to JoVE Core videos
Q1: What is a physical quantity and why does it need units?
A physical quantity is any number that describes a physical phenomenon quantitatively, such as length or mass. Units are essential because they provide standardized context for measurements. Without units, a number is meaningless—reporting the Eiffel Tower's height as 300 without specifying meters or feet leaves the measurement ambiguous and difficult to interpret.
Q2: Why was the International System of Units developed?
The International System of Units, or SI units, was developed to establish a common measurement system for accurately reporting physical properties of matter. This standardized approach ensures that measurements are universally understood and comparable across different contexts and countries, eliminating confusion from using multiple incompatible measurement systems.
Q3: What are the two main systems of units used in practice?
The two primary systems are the metric system, which uses SI units and is the international standard adopted by scientists and mathematicians worldwide, and the customary or imperial system, which uses English units. SI is the most widely used globally and is considered the standard for scientific work.
Q4: How are derived quantities different from base quantities?
Base quantities are fundamental measurements like mass, length, and time, while derived quantities are calculated from base quantities. For example, volume is derived by cubing the side length, and density is calculated by dividing mass by volume. Understanding base quantities and derived quantities helps organize how all physical measurements relate to one another.
Q5: How can you define a physical quantity using measurement methods?
A physical quantity can be defined by specifying its measurement method. For instance, mass can be defined using a weighing machine, and length using a meter scale. This approach directly connects the abstract concept to practical measurement tools, making the quantity concrete and reproducible in any laboratory or field setting.
Q6: Why is choosing appropriate units important for different measurements?
Choosing appropriate units depends on the size of what you're measuring. A cube's side length might be expressed in meters for large objects or centimeters for smaller ones. Using the right unit scale makes measurements practical and easy to communicate, preventing awkwardly large or small numbers that obscure the actual magnitude of the quantity.
Q7: What does SI stand for and why is it the global standard?
SI stands for Système International d'Unités, the International System of Units. It is the global standard because it provides a unified, internationally recognized framework for measurement that scientists and mathematicians worldwide accept and use. This universal adoption ensures consistency and facilitates communication across all scientific disciplines and countries.