# The Difference Between Mass and Weight

Kids are naturally curious, so teaching them the difference between mass and weight is a great idea. It can be confusing, however, when many common objects are referred to by both terms.

For example, a person’s weight is the force of gravity on their body, but their mass is the number of atoms they contain.

## What Is Mass?

Mass is a property of matter, and it measures the quantity of matter in an object. It’s the number of atoms and their density that give objects their mass, not their size or shape. This is why two objects of the same size can have different masses. It’s also why bricks are heavier than feathers, even though they have the same amount of matter.

Newton’s law of universal gravitation states that an object’s weight (w) is proportional to its mass (m), and that force equals mass times acceleration. So, if you push two objects with the same magnitude of force, the one with more mass will accelerate faster.

Weight is a vector quantity, and it changes depending on the object’s position on Earth, which is why you weigh less on the Moon than here on Earth. However, an object’s mass stays the same in space. This is why people sometimes confuse mass with weight. In fact, the word mass is related to a Christian church ceremony in which bread and wine are consumed.

## What Instrument Is Used to Measure Mass?

There are several instruments that can be used to measure mass. The most common is a physical balance. This is a system of levers with scale pans on each side. The unknown mass is positioned over one of the scale pans and then standard masses are placed on the opposite side so that the pointer rests at zero when the lever is raised.

Another type of mass measuring instrument is a pan balance, which is very similar to the scales you would find in a grocery store. These instruments are inexpensive and easy to use. For more precise measurements, scientists prefer to use transducers that can measure liquid properties in a static state. These instruments send a signal to the processor, which makes further mass calculations and then displays the mass on the indicator.

Besides these instruments, it is also possible to measure mass directly by calculating the center of gravity and moment of inertia of an object using known quantities that have themselves been derived from instruments. For very large objects, such as stars, this is done indirectly by observing their gravitational interaction with other massive objects in space.

## What Is Weight?

Many people confuse mass and weight, but they are not the same. Mass is an inertial property of matter, while weight is the force of gravity exerted on an object.

The formula for weight is W = mg, where m is the mass of the body and g is the acceleration due to gravity. Unlike the kilogram (kg), which is an SI unit, the kilogram-force and dyne are non-SI units of force.

A physical object’s weight varies depending on the location of the object. A 15-gram bird would have a different weight on the Moon than it does on Earth, because of the varying strength of gravity. In general, the heavier an object is, the greater its mass. A weighing scale indirectly measures mass, comparing an object to references such as reference objects or the International Prototype Kilogram kept at NIST. An ounce, for example, is a measure of an object’s weight. However, the term “weight” should be phased out and replaced with the more accurate term, “matter,” when referring to the property of matter.

## What Is the Metric System?

The metric system is the measuring system used by most countries. Only Liberia, Myanmar and the United States don’t use the metric system. A meter is the unit of length, kilograms are the units of mass and degrees Celsius are the unit of temperature.

The development of the metric system began during the Age of Enlightenment when leaders in France realized how complicated their previous measuring system was. They streamlined it and based measurements on natural principles and the power of 10.

The basic units that form the metric system are known as base units because they represent fundamental orthogonal dimensions that correspond to how we perceive nature: one for space, another for time and finally, one for inertia and another for electricity or electromagnetism. Other perceptual quantities like area and volume were derived from these base units using logical relationships. They are also identified by a set of prefixes that relates them to the base units.