# The Difference Between Mass and Weight

Many people use the terms weight and mass interchangeably, but they are two distinct physical properties. Mass represents the amount of matter an object contains, while weight depends on the force exerted by gravity on that object.

The most common unit used to measure mass is the kilogram, abbreviated as kg. There are other units of measurement for mass, however.

## Units of Mass

There are many units of mass used in the Metric System of Measurements (MSM), but the most common is the kilogram, or kg. A kilogram is the equivalent to 1000 g.

The kilogram is one of the seven base units of the SI, or International System of Units. It is defined by the mass of the International Prototype Kilogram, a roughly golfball-sized platinum-iridium cylinder stored in a vault in the International Bureau of Weights and Measures on the outskirts of Paris.

There is an effort underway to redefine the kilogram in terms of a fundamental physical constant, similar to the way the meter was redefined in terms of the speed of light. Two possibilities are currently being investigated: the Planck constant and the Avogadro constant.

## Gravitational Mass

It’s obvious from Galileo’s Pisa experiments (Figure 5.3) that the strength of an object’s gravitational force depends only on one property: its mass. But what isn’t so obvious is the fact that the inertial property of an object, its resistance to acceleration, also depends on its mass.

This is what led Einstein to develop his weak equivalence principle — that inertial and gravitational masses are the same for all objects and substances. This is why, for example, a heavy brass cylinder and an aluminum cylinder of the same size are both heavier than air and have the same weight, even when they’re in space far from any significant source of gravity. However, this is not the case when comparing an object’s inertial and gravitational mass at rest, because they follow different trajectories. So, for this comparison, a true balance is used. This is the type of scale that you find in a bathroom or clinic that measures your weight.

## Inertial Mass

Mass is a property of an object that determines its resistance to change in motion. It is measured by applying a known force to an object and measuring the acceleration that results. An object with greater inertial mass will accelerate less than an object with lesser inertial mass when acted upon by the same force, because it requires more force to cause a given acceleration.

In contrast, an object’s gravitational mass is determined by the net force and acceleration of that object in a given gravity field. These two measures of mass differ and some physicists make a distinction between them, with some using scales and true balances to measure gravitational mass and others dispenseing with gravity altogether, like astronauts aboard Skylab, by counting falling atoms.

Most digital scales give you a weight number for an object, but they aren’t necessarily measuring either the inertial or gravitational mass of that object. The reason is that the same unit of measurement — kilograms (kg) — is used for both types of mass, because they are proportional.

## Weight

The terms weight and mass are often used interchangeably, especially outside of physics, but they are actually two different physical measurements. Mass is a measure of matter that an object contains, while weight is the gravitational force that an object feels due to gravity.

An object’s weight is determined by how much force is needed to accelerate it, and the more mass an object has, the more it will resist acceleration. For example, a small kitten has very little mass, so it can be moved with a relatively light force. An elephant, on the other hand, has a lot of mass, so it will take a significant amount of force to move it.

In some occupations, such as chemistry and metallurgy, it is important to know the difference between mass and weight so that specific applications are correctly addressed. In other cases, such as in commerce and common usage, the words can be interchanged. However, it is advisable to phase out the use of weight in favor of mass whenever possible.

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