Many children have an inquisitive nature, so it’s a good idea to introduce them to the concept of mass early. Mass measurement is vital to industry and scientific research, from weighing scales to complex instruments like mass spectrometry.
Mass is usually measured by comparing an object to a standard, known mass, such as the NIST prototype kilogram. But there are also other ways to measure mass without using weight.
Weight
In a uniform gravitational field, an object’s mass determines its force of attraction to itself and other objects. The force exerted by an object is called its weight (or gravity) and can be measured with a simple weighing scale.
Unlike the other 7 SI base units, which are all derived from an invariant of nature, the kilogram remains tied to a physical prototype. The International Prototype Kilogram (IPK) is a solid platinum-iridium cylinder housed in a safe at the BIPM in Sevres, France. Replicas of the IPK are distributed to CGPM signatories who request them, along with calibration certificates based on comparisons to the IPK.
Although Newtonian physics made a clear distinction between mass and weight, the word weight continued to be used when it should be replaced by the more accurate term mass. As a result, the 3rd CGPM in 1901 officially defined weight as the product of an object’s mass and its gravitational acceleration.
Gravity
Gravity, the force that causes objects to attract each other, is an essential part of mass. It is the most important force that affects objects and people on Earth.
It took a flash of insight from Isaac Newton to transform gravity from an inscrutable tendency of objects to fall toward the ground into a measurable and predictable phenomenon. Today, gravity is one of the four fundamental forces in physics.
Most of the time, the gravity field is pretty much the same everywhere on Earth. This is because large, stable features such as mountains and oceans have relatively consistent mass. These features do move slightly over time, but their changes are small enough that they can be ignored. Gravity can also vary slightly with location because of the rotation of the Earth. This variation is called active gravitational mass and is calculated by letting a test object fall freely and measuring its acceleration. It is typically a few milliseconds per mile and can be corrected for using the formula g = (G*(m1+m2)/rE2).
Force
While the terms force, mass and weight are often confused, they are distinct concepts. Mass is a fundamental quantity directly related to the number and type of atoms in an object, and does not change if the object moves or changes shape. Force, on the other hand, is a derived quantity that combines an object’s mass and its gravitational acceleration.
NMC mass standards are produced in stainless steel with a density similar to that of the primary standard kilogram at the International Bureau of Weights and Measures. This allows for an accurate buoyancy correction to be applied to the calibration of the mass standard, resulting in a calibrated weight that is valid anywhere in the world without having to make additional adjustments to account for local gravity and air density conditions.
The importance of accurate measurement of mass and force cannot be overstated. Accurate mass & force measurements are used in a wide range of industries including aerospace, construction, automotive, pharmaceutical and manufacturing, providing critical information on the quality of products & services; and performance of equipment & machinery.
Atoms
Atoms are the basic building blocks of matter and chemistry. They can combine with each other to form molecules and they can also ionize to form ions. The study of atoms is important because they are the smallest particles that can be made up of matter and cannot be divided further into smaller parts by normal chemical processes.
Atoms consist of three kinds of subatomic particles: protons, neutrons and electrons. Protons and neutrons have a great deal of mass and are found in the nucleus of an atom while electrons occupy the space around them. Protons carry a positive charge while electrons have a negative charge.
The atomic number, which is the sum of the numbers of protons and neutrons in an atom, defines the mass of that atom. The atomic mass unit (u) was once used to measure the atomic masses of substances; however, because the atomic weights of most natural samples are not identical, the atomic weight is now more commonly measured using the dalton.