Many people are confused about the differences between mass, weight and force. This comic-book video animation series from the BBC explains what mass is, and how it differs from weight.
Children learn fastest in a relaxed atmosphere, so this is an excellent way to introduce the concept of mass. It’s important to understand that an object’s mass does not change with its shape, size or location – but its weight does depend on the strength of gravity in different places.
Definition
Mass, measured in kilograms (kg), is the amount of matter in an object. It is a measure of substance, unlike weight, which depends on the gravitational force of the Earth and can vary depending on where you are in the universe.
You might be surprised to learn that a pineapple and a baseball bat have the same mass! It is also important to understand the difference between mass and weight. Kids are naturally inquisitive, so introducing them to these concepts early will help them understand more difficult topics like mathematics and physics later on.
The international prototype kilogram, kept at NIST, is the standard unit for mass measurement. Other units that can be used for mass are grams and tonne. The kilogram is part of the metric system and an essential measurement for global trade.
Methods
Traditionally, very small masses and forces have been measured with sets of carefully calibrated metal weights. This approach is no longer practical for many modern applications such as manufacturing, pharmaceutical development and spacecraft sample-collection missions.
Alternatively, mass can be measured with a balance. The unknown mass of a body is contrasted against the known estimation of its mass to determine the value of an unknown force (F) required to produce that mass (m). A balance also works in space and places with no gravity as changes in gravity influence both masses on the scale equally.
NIST scientists are developing a highly accurate method of measuring extremely small masses and forces without calibration weights using radiation pressure that a laser beam exerts on an attached high-reflectivity mirror, balancing this force against an electrostatic force on a capacitor. This system is expected to be used as the new standard for the kilogram in 2018.
Accuracy
The most common way to measure mass is with a balance. We use balances all the time to weigh things. However, the scales don’t always work the same on different planets. That’s because of the forces of gravity that are acting on the mass.
Scientists and engineers know that there are two distinct concepts to consider when taking measurements: accuracy and precision. Accuracy describes how close a measurement is to the true value of a quantity, while precision refers to the degree to which repeated measurements under identical conditions show similar results.
In manufacturing, for example, high accuracy is important because it allows parts to fit together easily and smoothly. But consistency is more critical, so manufacturers often choose to focus on precision rather than accuracy.
Applications
Mass measurement is a vital part of daily life and technology. For example, hardware items are often sold by weight, influencing pricing and portioning. Food manufacturing and pharmaceuticals rely on precise measurements for quality control. Farmers adjust fertilizer applications based on mass analysis results.
Traditionally, the most common method of measuring mass is with a balance. A balance compares an unknown mass with a known one, reading the result as a force in newtons that counteracts gravity. This approach works in space and also in places where gravity is a factor.
Other techniques measure a sample’s change of mass as it goes through a deposition, etch, or clean process. Such dynamic measurements can detect shifts in chemical equilibria or reveal assembly/disassembly steps of complex molecules.
Future developments
In many fields of science, mass measurement is essential for accurate and repeatable experiments. It is the foundation of discoveries in physics, biology and chemistry. It also drives industrial efficiency, product quality and safety. For example, it is critical for the precise administration of drug dosages in healthcare, which impacts patient safety and treatment efficacy.
NIST scientists helped redefine the kilogram, the fundamental unit of mass in the International System of Units (SI), in 2019. The new definition replaces a physical object—a hunk of platinum-iridium alloy called Le Grand K that’s kept in France—and is based on fundamental properties of nature that are inherently stable over time.
Researchers are developing instruments to directly measure these constants, such as devices known as watt balances and methods that use atom-counting techniques like charge detection mass spectrometry or nanoelectromechanical systems mass spectrometry. These developments will enable even more precise mass measurements.