Most scientists use the metric system for measurement. This system uses units such as the kilogram (abbreviated kg), which is the standard unit of mass.

You can measure mass with a balance-type scale or an electronic weighing instrument, or even by holding something on your hand. Mass is the amount of matter an object has and will be the same on Earth or in space.

## Measuring Mass

Mass is a measure of the amount of matter in a body. It determines inertia and resistance to change in velocity, and is important in Newton’s laws of motion and Einstein’s E = m c2 equation for energy. It is usually measured using a balance.

Unlike density, which depends on an object’s size and shape, mass depends only on the type of atoms and how many of them it has. Therefore, a pineapple has the same mass as an iron bar, even though they have different densities.

Because of its dependence on physical constants, mass measurement can be difficult. It is also subject to variations due to temperature, which is why a standard kilogram has been defined as the mass of a solid prototype made from platinum and iridium, kept in a standard lab. Researchers are working to define a new kilogram that is fixed to an invariant of nature, which would make the results more stable than those of the Paris prototype.

## Units of Measurement

Throughout history, many official systems of measurement have been in use. These range from the metric system in most countries to customary units still used for some day-to-day measurements, such as pounds and ounces.

For example, the unit for length in metric systems is the meter, while the units for length in customary systems are feet and inches. Units for other physical quantities, such as volume, can also be derived from these fundamental units. For instance, the liter (L) is defined as the volume occupied by a cube with sides of 1m1m on each side.

A specialized unit called the kilogram, kg, is part of the metric system and is used in all countries that have adopted the metric system. For example, the primary prototype kilogram in the United States is a platinum-iridium cylinder held at the National Institute of Standards and Technology (NIST). It replaced a physical prototype of the unit in 1875. The kilogram and the metric system are often referred to as SI units, or Standard International System of Units.

## Conversions

As the world becomes more and more global, it is important to be able to convert between different systems of measurement. For example, while most people in the United States still buy gasoline by the gallon and buy apples by the pound, many companies are now selling their goods to countries that use the metric system of measurement. In these countries, the prices for things like bananas and medicines are displayed in kilograms (kg), gram (g) and milligrams (mg).

To change from one set of units to another, a conversion factor must be used. For example, to convert from inches to centimeters, you must know that 1 inch equals 2.54 cm. When converting from customary to metric, a common method of unit conversion is called dimensional analysis. This method works by comparing the numerator and denominator of the fraction to see if they cancel. Click here for a dimensional analysis applet. It also covers units of volume and capacity.

## Applications

Mass measurement is used for a variety of purposes, from petrochemistry to quality control of soft drinks and alcoholic beverages. It’s also used to measure the density of a liquid, which is a standard property for determining how a fluid will react once it’s been injected into an engine.

The most common method of measuring mass is with a balance. The object to be weighed is placed on one side of the balance, while an equal amount of weight is put on the other side. This allows the known value of mass to be measured by comparing it to the unknown mass.

More sophisticated methods of measuring mass are available, such as mass spectrometry. This analytical technique identifies molecules by their molecular weight, then calculates the exact mass of each component in the sample. More recent methods such as orthogonal acceleration-time of flight (oa-TOF) mass spectrometry make exact mass measurements easier and more affordable. These techniques allow scientists to identify unknown compounds as well as quantify known ones.