Mass is the quantity of matter in an object. A heavy object has more matter in it than a light one, so it has greater mass.
The most common unit for mass is the kilogram, which is 1000 grams (g). Students can learn about the 7 SI base measurement units with this comic book-style video.
Definition
The most basic property of matter is its mass. An object’s weight varies with its environment because of the gravitational pull of the Earth on it. A heavy object has more matter in it than a light one, so it has greater mass.
However, the density of the object is a factor as well, and differs between objects with the same mass. In fact, a scale can be calibrated to produce different results depending on whether the mass is measured against a stainless steel standard or an air standard.
The SI unit for measuring mass is the kilogram (kg), which was first characterized in 1795 as one cubic decimeter of water at the melting point of ice. One kilogram is equal to 1000 grams. The atomic mass unit is a more precise way to measure an atom’s or particle’s mass; it is defined as 1/12th the mass of a Carbon-12 atom. This measurement method allows for a more accurate calculation of mass in cases where the physical prototype of a kilogram is not available.
Units
The units of mass are kilograms (kg) and grams in the metric system and pounds or ounces in the US customary and British imperial systems. Weight is a measure of the inertial property of matter and depends on an object’s location, so it can change with its environment.
All measuring systems have basic units of length, mass, capacity (volume), and temperature. These are called the fundamental or base measurement units. They are defined for only these physical quantities. Other measurements are derived from these, such as velocity, which is determined by dividing a distance quantity by a time quantity, like meters per second or micrometers per nanosecond.
The standard unit of mass in the metric system is the kilogram, which is defined as the mass of a specific artifact in Paris known as the International Prototype Kilogram, or kg. This artifact is maintained by the BIPM, and copies of it are distributed to countries that have signed a diplomatic treaty to become members of the metric system.
Conversions
Some physical quantities have multiple units that can be used to describe them. Conversions between these units occur frequently. These conversions can be made using unit ratios and dimensional analysis. For example, converting inches to centimeters involves multiplying the number of inches by the conversion factor of 10 and then dividing by 100.
The most common types of conversions involve mass and weight measurements. While the United States continues to largely use customary units, many other countries have adopted the International System of Units (metric/SI) of measurement. Therefore, it is important for people across the globe to be able to communicate with one another about these measurements. To help with this communication, this page contains many helpful links to conversion tables and other resources. Several of these resources also contain practice problems. This page also includes a handy applet that allows users to convert between metric and US Customary units. Simply enter the starting units in the upper left corner and the desired units in the lower left corner, then click “go”. This applet also allows users to make mass conversions from liters to quarts.
Applications
Mass measurement is crucial to a variety of applications, from food inspection and quality control to drug testing and discovery, carbon dating, isotope ratio determination and forensic toxicology. It is particularly important in proteomics, where accurate masses are critical for protein identification and for delineating cellular networks and pathways in systems biology studies.
During mass photometry, the light scattered by a sample molecule in contact with a measurement surface interferes with the light reflected by that surface and is measured; this interference signal scales linearly with molecular mass. The resulting mass value is then compared with a known estimate of mass to calculate the sample’s gross mass.
Proteomics applications that rely on accurate mass measurements benefit from rapid developments in both MS instrumentation and databases. These include protein identification by matching parent ions or fragment ions with theoretical masses based on the sequences of proteins in genomic databases, as well as the detection of posttranslational modifications.