Most people use the words “mass” and “weight” interchangeably. In physics, however, they have very different meanings.

Mass is a property of matter that determines many other physical properties such as the strength of its gravitational force, inertia and resistance to acceleration. It is measured in kilograms and decimal multiples and fractions of the kilogram.

Weight

Although we use the words “weight” and “mass” interchangeably, they are two distinct physical properties. Mass is a property of matter, directly related to the number and type of atoms that comprise it. Weight, on the other hand, is the force created when an object’s mass is acted upon by gravity.

An object’s weight varies with its location in the universe. For example, a 15-gram bird will weigh differently on Mars than it does on Earth because the gravitational pull of the planet is different.

An object’s weight is measured by a balance scale or by comparing it to references, such as a box of apples. The unit used to measure an object’s weight is the kilogram. However, smaller objects are often measured in grams. The kilogram was re-imagined in 2019 to be based on fundamental constants of nature, making it more accurate than ever before. The atomic mass unit (amu) is also now a standard measurement of mass.

Mass

Although the terms “mass” and “weight” are used interchangeably in everyday use, they are two completely different physical properties. While mass describes the amount of matter (or “stuff”) in an object, weight is a measure of the force of gravity acting on that object.

The measurement of mass is made using a balance that compares the unknown quantity with a known reference quantity, usually the International Prototype Kilogram (IPK). A national standard kilogram is then used to disseminate masses to multiples and submultiples of the kilogram, covering a range from 1 mg to 5000 kg.

Kids are innately curious, and it’s in their best interest to learn how much matter is contained in objects at an early age. It will help them effortlessly grasp concepts like mass and matter later in life when they’re studying physics, math, and other subjects. To start, explain that an object’s mass doesn’t change shape or size, but its weight does.

Force

Force is the magnitude of a weight-generating reaction or the amount of effort needed to accelerate an object. It is not the same as mass, even though it is often mistakenly used as such. Force is a vector quantity; mass is a scalar quantity.

Engineers and scientists in disciplines that use weighing systems understand the distinction between mass and force. For instance, an astronaut in weightlessness weighs less than on Earth because of lower gravity, but has the same mass.

The NIST Force Laboratory maintains national and international standards for measurement of very small masses, forces, and accelerations. In addition, the lab conducts pattern evaluation tests of load cells used in weighing systems to determine their linearity, hysteresis, repeatability and creep characteristics over a range of temperatures. It also performs deadweight loading tests to evaluate prototype load cell families used by weights and measures officials in certifying commercial scales. The lab is automated, enabling more complex testing and shorter test times.

Acceleration

Acceleration measures the change in velocity over time. As with displacement and velocity, acceleration has both magnitude and direction, so it is a vector quantity.

Sports announcers often say that someone is “accelerating” when they are increasing their speed, but acceleration has much more to do with changing how fast an object moves than it does with just getting faster and faster. Even an object with a constant velocity can be said to be accelerating if its velocity changes over time.

NIST has made significant improvements to the measurement of mass, including the development and maintenance of check standards with known masses. These are incorporated into weighing designs to ensure that the results of a weight are valid. This monitoring is based on the principle of the T-test, in which two artifacts with different but equal known masses are compared in a controlled environment to detect and correct for errors. In addition, NIST has developed and maintains facilities for hydrostatic [10,11] and immersed [9] solid density measurement of mass standards.