Weighing objects is one way to measure their mass. A simple spring scale that takes gravity into account calculates the force of attraction and solves for mass.

Many people use the terms weight and mass interchangeably, but they are actually different measurements. Weight depends on the gravitational pull of Earth, while mass is constant no matter where you are in the Universe.

## Weight

The gram (g) is the unit of mass in the metric system and in the centimeter-gram-second (CGS) system of units. It is also the smallest amount of substance that can be measured in a balance-type scale or on a load cell. Its abbreviation is g, although it can be seen spelled out in some contexts — for example, on legal-type product labels and in some regulated areas.

While the distinction between weight and mass is not important for most everyday activities – such as standing on a bathroom scale – it is crucial in science. Weight depends on the force of gravity, which varies in strength from place to place, while mass is a consistent measure of matter. In fact, your weight would change if you were on the moon or in space because the strength of gravity there is less. The same applies to the different planets of our solar system. The atomic mass of an element can vary, but the total mass will be the same.

## Mass

The accuracy of mass measurements is crucial in scientific research. Whether it’s measuring the ingredients in a recipe or determining the mass of celestial bodies, accurate measurements are critical. Traditionally, these measurements have been made using balances and other types of scales. These devices are also essential in teaching students about the physical world and how to make precise measurements.

Children learn faster when they are engaged, so introducing them to concepts like gravity at an early age is key. It can be fun to compare the weight of a pen and bottle of water. Children can then explore why a heavier object feels heavier than a lighter one.

When measuring mass, it is important to remember that the measurement is a scalar quantity; in other words, it does not change according to location. NIST’s Kibble balance is a new type of mass standard that does not rely on physical weights to perform calibrations directly traceable to the kilogram, the metric unit of mass.

## Force

In science it is important to understand the difference between mass and force. Many people use the words weight and mass interchangeably as if they were the same thing, but they are not. Mass is the amount of matter an object contains, whereas weight is the force of gravity on that matter.

Mass is measured using a balance scale. An unknown object on one side of the balance must exactly balance a standard kilogram mass on the other side for the balance to say the object has a certain mass.

The standard kilogram is defined as the specific fraction of a platinum-iridium cylinder kept at the International Bureau of Weights and Measures in Sevres, France. This is also the definition used by NIST in the USA. Stainless steel mass standards have an error of about 150 ppm. In engineering, where there is often a need to calculate the forces on structures due to gravity, engineers convert the mass of the structure into a force in newtons by multiplying it by a factor that takes into account local differences in the rate of gravitational acceleration.

## Gravity

For centuries, humanity struggled to understand the force that pulled objects down. It took a flash of genius from Newton to elevate gravity from an inscrutable tendency of things like raindrops and cannonballs toward the ground to a predictable, measurable phenomenon.

Gravity is what holds your feet to the floor when you stand on a scale and what keeps the International Space Station floating in orbit. It’s also what makes a clock aboard an orbiting satellite advance faster than one on Earth, a phenomenon known as gravitational time dilation.

The strength of gravity at any location on Earth is determined by an equation most students learn in high school: the acceleration of an object (symbolized as g) is proportional to its mass and inversely proportional to the square of the distance between the center of mass of the two objects. But standard gravity values differ slightly from location to location, due to factors such as latitude, elevation above sea level (the geoid), geology, isostasy and subsurface density variations.