Mass is the amount of matter something has. Things made of matter are everywhere — the screen you’re reading this on, your keyboard, and the air you breathe have mass.

People measure mass all the time using a balance. A balance works the same on Earth as it does on the moon or any other planet, since gravity doesn’t change the results.

Weight

The most common way of measuring mass is to use a balance or spring scale. A balance is unaffected by gravity, and the comparison it makes between an unknown object and a known object enables the measurement of its weight, in units called kilograms, which belong to the metric system, not the less-popular U.S. customary or British Imperial system.

The word weight is used in physics to describe the gravitational attraction between two objects, but many introductory physics textbooks use it instead of the more correct term “mass.” This historical misuse led to the 3rd General Conference on Weights and Measures defining weight as the product of an object’s mass and the gravitational acceleration of its location.

The gram, a fundamental unit of mass in the centimeter-gram-second (CGS) and later meter-kilogram-second (MKS) systems of measurement, is defined as being almost exactly equal to the mass of one cubic centimeter of pure water at standard temperature and pressure. This definition means that high-precision balances in routine laboratory use can be calibrated to conventional mass, or true mass minus an unknown degree of buoyancy.

Gravity

Gravity is the most important force we notice, determining the motion of planets, stars and galaxies. It also acts on us, pulling us toward the ground when we stand up or causing our bodies to orbit around the Earth and stars.

In general, the more mass an object has, the stronger its gravity will be. This explains why a large block feels heavier than a smaller one. It also explains why balance-type bathroom scales are unaffected by gravitational acceleration, while load-cell-based scales depend on it.

While we usually use the words “weight” and “mass” interchangeably, scientists differentiate them. Mass is a property of matter: it depends only on the type and number of its atoms. Weight is the result of Newtonian gravity: it varies depending on location, and it can be measured using gravimetry. In the International System of Units, mass is measured in kilograms; in the U.S. customary system, it is measured in avoirdupois pounds. Engineers in disciplines involving weight loading (the force on an object due to gravity) convert mass into newtons and pascals, units of force and pressure respectively.

Density

Density measurement is important in a number of scientific, educational and industrial contexts. For example, anyone who has ever tried to mix water and oil in a salad dressing bottle will have witnessed the effect of density first-hand (the oil floats on the water).

In chemistry, the calculation for density helps scientists predict how substances will interact under specific conditions. In geology, density helps researchers understand how different rock and magma layers may cause seismic activity. For scientists working with materials and chemicals, accurate measurements are essential for ensuring quality control.

There are many different methods for measuring density, but the process always begins by identifying the mass of a sample and determining its volume. Then, the equation for density can be calculated – mass divided by volume. The units for mass and volume vary depending on the context, but they can be as simple as kilograms per cubic meter (kg/m3) or grams per cubic centimetre (g/cm3).

Force

Measurement of force is crucial to aerospace, automotive, construction, electronics, manufacturing and pharmaceutical industries. It can help engineers to develop better materials and structures, as well as improve existing designs. It is also important for sports, allowing researchers to optimize training and equipment and identify weaknesses in athletes’ technique.

It is a property of an object that dictates the strength of its gravitational attraction with other bodies and its resistance to acceleration (change in velocity). In modern physics, it is determined by measuring an object’s inertial mass: An object with smaller inertial mass accelerates more rapidly than an object with greater inertial mass.

It is measured by using a balance to compare the obscure mass of an object with a known estimation of its mass. The units used are kilogram-force and newton. One kilogram-force is equal to approximately 0.45 Newtons. The unit newton is named after Sir Isaac Newton and combines the fundamental SI units of length, mass and time into one.