# Mass Measurement

Mass is a fundamental property of matter. It determines an object’s resistance to acceleration when net force is applied.

Measuring mass involves comparing an unknown object to objects of known mass. Typically, this is done using balances and scales, but other tools exist. In space, scientists use inertial balances to find an object’s mass.

## Definition

In physics, mass is the quantitative measure of inertia, which is an object’s resistance to change in its speed or position caused by external forces. The greater the mass of an object, the more it weighs.

Nevertheless, in everyday life the terms “mass” and “weight” are frequently used interchangeably. For instance, in retail commerce, items are labeled with a net weight that refers to mass (grams and ounces). The term “weight” is also used to describe the force of gravity on an object.

The most common way to measure mass is using a balance. The unknown mass of a body is contrasted against a known value of mass to obtain the estimation of the unknown mass. This method works in space and places of no gravity as well since changes in gravity affect both masses on the balance equally. One kilogram is the standard unit of measurement for mass. One kilogram is equal to the fixed numerical value of Planck’s constant h, which is defined as 6.62607015 x 1034 joule seconds.

## Units

Units of measurement are used to quantify physical quantities. The units of mass, length and volume are commonly used in the metric system which is the standard measurement system worldwide. These are called the SI (Systeme International d’unités) base units and include the meter, kilogram, second, ampere, kelvin and mole.

The basic metric unit of mass is the gram, which is equal to about one teaspoon of sugar. A kilogram is about 2.2 United States pounds. The basic metric unit of length is the meter, which is about 3 feet long. A liter is slightly larger than a quart.

All metric measurements are based on powers of 10. Each derived unit is 10 times larger than the base unit, which makes converting one metric measurement to another a straightforward process. For example, a liter is equal to the volume of a cube that measures 1cm1cm on each side. This is a very large cube, but for everyday use, the liter is defined to be 1000cm3 or 1dm31dm3. The names of metric units are formed by attaching prefixes to these base units.

## Scales

Scales can be used to measure mass in a variety of settings. For example, a person’s weight can be measured by standing on a digital scale or using Sir Isaac Newton’s second law of motion (force equals mass times acceleration) to determine the force exerted on the person by gravity.

More sophisticated weighing instruments such as analytical balances measure mass by directly comparing an unknown quantity to a known quantity, eliminating the need for assumptions about gravity. However, these instruments are typically not used in the home.

When using a commercial scale at home or work, it is important to press the tare button on the instrument before adding any objects for measurement. This will eliminate the weight of the container from the final measurement and make it more accurate. An evaluation of 233 dial and digital scales from primary care, diabetology and endocrinology clinics as well as fitness and weight loss centers found that about 17% of the scales had a precision error greater than 2.7 kg or about 1 Body Mass Index (BMI) unit [10].

## Methods

Traditionally, mass measurement has been made using a balance. This compares the obscure mass with a known estimation of its weight and determines its value. It works well enough, but changes in gravity influence it and other factors such as temperature, evaporation, vibrations etc. Therefore, a system that measures masses online and independently of these influencing factors is needed.

Modern mass spectrometer software reports accurate mass measurements to four decimal places and sometimes more for masses below 10 mDa. However, rounding errors will occur if the number of measurements is not sufficiently large. Therefore, when reporting results, it is advisable to report them to at least one decimal place (i.e. significant) to reduce the possibility of error due to rounding. Similarly, the root mean square error (RMSE) of an accurate mass measurement will vary inversely with the square root of the number of measurements, and must be carefully calculated. The RMSE will also depend on the strength of the signal, the ionization technique and the background noise level.

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