While many people use the terms weight and mass interchangeably, they are different measurements that describe heaviness. Mass is a property of matter that determines the strength of its gravitational attraction to other objects and its resistance to acceleration.
The earliest measurement devices were balance scales that compare masses by placing known masses on each side of the balance. The unknown mass is then calculated as the difference between the known masses.
Quantity
In physics, mass is a physical quantity that measures the amount of matter in an object or particle. It is an unchanging quantity, although it can be influenced by the conversion of mass into energy and vice versa. It is measured in kilograms and related units.
A distinction should be made between mass and volume, which is the measure of three-dimensional space or the shape that an object or substance takes up. For example, a basketball and a bowling ball have similar volumes but differ in their masses.
To determine an unknown mass, the simplest method is to compare it to a known mass. This requires the use of a balance, which is typically calibrated with the standard kilogram (a copper prototype in 1795, the platinum Kilogramme de Trace in 1889, and the current international prototype, the metric kilogram) to ensure accuracy. This procedure is time consuming, costly, and subject to mechanical failure and loss, but is still used in most legal applications such as for air transportation or international entry.
Force
Many people use the words mass and weight interchangeably. While they are related, it is important to understand that they measure different physical quantities. Mass is an intrinsic property of matter, while weight depends on the force of gravity at a particular location.
When you pick up a pen and a bottle of water, the bottle feels heavier because it has more mass. Using this example is an easy way to broach the topic of mass and explain how it is related to weight.
The kilogram, a fundamental unit in the International System of Units (SI), is a standard of mass. It was established in 1875 at the International Metric Convention, which also created the meter and second as SI units. The prototype kilogram is stored in a platinum-iridium cylinder at the International Bureau of Weights and Measures in Sevres, France. Mass & Force Laboratory offers training on these and other metrology quantities to technical professionals and managers.
Density
The property that determines how much an object weighs for its size, density is a fundamental measure of matter. It is defined as mass divided by volume and is commonly measured in kilograms per cubic meter (kg/m3) or kilograms per cubic centimeter (g/cm3).
Because all materials have different compositions and makeups, each has its own specific densities. Knowing the densities of two materials provides information that may be useful in separation techniques such as separating oil from water. It is also the reason why ships and submarines float on water, while cars sink in it.
Density is directly proportional to temperature and indirectly proportional to pressure. It is important to understand this relationship so that the proper air value can be found for manual instruments such as a pycnometer during calibration. For example, pure water and ethanol have the same density at 20 degrees Celsius but ethanol has a higher density than water when at a lower temperature.
Inertia
The measurement of mass at the atomic and molecular level continues to be critical for a wide variety of scientific analyses. Spectroscopic techniques, chemical kinetics, and others rely on the knowledge of atomic masses to determine molecular composition and behavior.
Inertia is the tendency of an object to resist changes in its state of motion. This property varies with the object’s mass, so that a bigger, more massive object tends to resist change in its motion more than a smaller, less-massy object.
Galileo used a series of experiments to demonstrate inertia. He observed that a ball dropped down one plane and up the other would stop at an equal height if friction were eliminated. He therefore viewed the phenomenon as an “innate force” inherent in matter that resists acceleration, which became synonymous with inertia.
In modern physics, mass and energy are related by the equivalence principle in Einstein’s theory of relativity. This principle states that the total mass of an accelerated observer is equivalent to the gravitational force experienced by that observer in its inertial frame of reference.