# How to Use a Mass Measurement Scale

Mass is a property of matter. It doesn’t change with changes in shape or position. It’s the same on Earth, on Jupiter or in space.

It is important to understand the difference between mass and weight. Many people use these terms interchangeably but it is a mistake. Mass is a fundamental physical property and weight is only a measure of how an object interacts with gravity.

## Definition

Mass is a measure of the amount of matter an object contains, and it is measured in kilograms and related units. Weight, on the other hand, is a measurement of the force exerted on an object by gravity on Earth, and would be different if you were standing on another planet.

In physics, mass is also the quantitative measure of an object’s inertia – the resistance it offers to acceleration (change of velocity) when a force is applied. Thus, objects of the same mass have the same inertia and the same acceleration under identical conditions.

When using SI units, the word “mass” is used rather than the term “weight.” However, many people still use the term weight to refer to an object’s inertia and the force of gravity acting on it. This is confusing, and the terms should be distinguished as described below. The meter and the kilogram are two of the seven base units of the International System of Units (SI). Countries that subscribe to the metric convention were given copies of the international standard artefacts – a copper prototype of the metre, and a platinum-iridium International Prototype Kilogram – which they maintain, known as National Prototype Metres and National Prototype Kilograms. These are compared against the international standard at regular intervals.

## Units

The SI comprises a coherent system of seven base units: the second (s, unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), candela (cd, luminous intensity) and mole (mol, amount of substance). It can also accommodate coherent derived units, which are obtained as products or ratios of base units.

For most of the base units the BIPM publishes a mises en pratique, or “realisations” — in other words, the best practical realisations currently available. This separation of the defining constants from the definitions means that improved realisations can be developed as science and technology advances, without needing to redefine the underlying units.

The kilogram is the only one of the base units to be defined by a physical artefact; its physical properties are constantly changing, so international scientists have been urging a redefinition based on some other invariant property. Two possibilities have attracted particular attention: the Planck constant and the Avogadro constant.

## Examples

In order to measure mass, you need a balance scale. Put the object you want to weigh on one side of the scale and add weights or other objects that have the same amount of matter to the other side. The amount of weight needed to counter the force of gravity is the object’s mass.

Alternatively, you can use an electronic mass measurement device. These devices have sensors that can detect the movement of atoms or molecules, which can then give an estimate of an object’s mass.

While it might be tempting to confuse weight and mass, it is important to know that they are two different measurements. Objects may be weightless on the moon due to lack of gravity, but that doesn’t mean they don’t have mass. For example, a rock may have the same mass on the moon as it does on Earth. However, the moon is much smaller than Earth, so it would take less weight to equal the same amount of matter on the moon.

## Applications

A balance or scale is one of the most common instruments used to measure mass in a science laboratory. A high-precision scale calibrated with stainless steel standards translates the force exerted on an object by gravity into its conventional mass (true mass minus 150 ppm of buoyancy).

The same technique can be used for single-molecule mass measurements. By measuring the frequency at which ions are emitted from an Orbitrap instrument, a software program plots what Thermo calls selective temporal overview of resonant ions (STORI) data.

Benesch, for example, uses this information to study assemblies made between molecular chaperones and the proteins they protect in cells. He wants to know whether these structures prevent the formation of amyloid fibers, a protein aggregate that’s associated with eye problems and other diseases. Using mass photometry, he can see whether a complex has assembled properly or clumped together into a glob of debris. Having this information can save time and effort, because researchers can immediately filter out samples that aren’t suitable for other structural biology methods.

Posted in News.