# The Basics of Mass Measurement

Mass measurement is fundamental to many scientific disciplines. The most common way of measuring mass is by using a balance. The kilogram, or kg, is the standard unit of mass in the metric system. It was originally based on the physical prototype, but has since been changed to better reflect modern physics.

## Units

If you are a teacher, it is important to familiarize your students with units of measurement. In particular, mass is an important concept for them to understand. It can be measured in a variety of ways, but the most appropriate unit for a particular situation depends on the size and mass of the object. For example, a car is much larger than a paper clip, and it should be weighed in kilograms rather than milligrams or grams.

To make sense of these units, students need to learn about the underlying physical constants that they are based upon. This is why scientists have separated the defining constants from the units themselves. In doing so, improvements to measurements can be made and the mises en pratique (French for “putting into practice”) updated without having to change the definition of a base unit such as the kilogram.

The kilogram is the only SI base unit whose name and symbol include a prefix. This is due to historical reasons. The BIPM has been discussing changing this for some time, however. The proposed change would fix the kilogram to an invariant of nature, such as the Planck constant or Avogadro’s constant.

## Gravitational force

Gravity is a force that pulls all objects that have mass down towards the center of Earth. The strength of this force is inversely proportional to the square of the distance between two masses. Its value, which is usually given as g, is 9.807 m/s2.

This value can be calculated from the ratio of the masses of two objects and their separation distance. But measuring gravity itself is difficult because it requires an instrument that can detect tiny forces.

Scientists have used different methods to measure gravity, but the most accurate one is a system known as atom interferometry. This technique uses laser light to split atoms of cesium into a state of superposition. This means that one version of the atom exists in two places at once and can be distinguished by atomic clocks.

Another way to measure gravity is to use a torsion balance. This device consists of two masses, one on an outer carousel and the other on a disk suspended by a fiber. The movements of the larger masses cause the disk to twist back and forth, which can be measured by a sensitive optical sensor.

## Inertia

Inertia is the tendency of physical objects to remain at rest or move at a constant velocity unless acted upon by another force. It is a key part of Sir Isaac Newton’s First Law of Motion, which states that an object at rest will stay at rest and an object in motion will continue to move at its current speed unless acted on by another force.

Inertial mass and gravitational mass are essentially the same thing. However, the distinction between them has been blurred over the years because no experiment has unambiguously demonstrated that gravitational mass is different from inertial mass.

In physics, mass is the quantitative measure of an object’s inertial property and how much matter it contains. It’s not the same as weight, which is a force that – along with gravity – affects how an object moves. For example, an object’s weight will change if it is moved to the Moon or the Sun, but it will still have the same mass.

## Weight

Despite the fact that in everyday language people often use the words “weight” and “mass” interchangeably, the two are different physical properties. In physics, mass describes the quantity of matter an object contains and weight describes its force of gravitational attraction.

In the past, scientists measured an object’s mass by comparing it with an international prototype kilogram or “IPK.” The IPK was a puck-shaped cylinder made of lead and machined to exactly weigh 1 kilogram. It was a key piece of the metric system, which is the international standard measurement system used throughout the world today.

But now, a new definition of the kilogram has been adopted. The new definition is based on Planck’s constant, which means the kilogram will not be dependent on any physical artifact. The change will allow for more accurate measurements, and it will be better for scientific research that involves tiny amounts of medications or other materials requiring nanoscale precision. The 90-minute MontanaPBS film The Last Artifact, features scientists at NPL and around the world who worked to make this change happen.

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