In everyday speech, the words mass and weight are interchangeable, but in physics they are different. The specific definitions of mass and weight show why mass is the more useful quantity when you measure the amount of matter present. Weight is a force rather than a measure of the amount of matter, and your weight depends on the celestial body you measure it on. Mass, on the other hand, is the same no matter where you are.

Mass and weight describe different things. Mass is the measure of the amount of matter in a specific thing, whereas weight is a measure of the force of gravity acting on that mass. The definition of weight states that:

W = mg

Here, W represents weight, m is mass and g is the acceleration due to gravity. Weight is measured in Newtons because it is a force. Note that the expression above is effectively the same as Newton’s second law: F = ma.

Mass is measured in kilograms, and although it’s difficult to define, the inertial mass of an object is given by:

m = F/a

Where F is force and a is acceleration. This shows mass to be an object’s resistance to motion when a force is applied. Mass is the fundamental property held by matter that is used in physics equations such as Newton’s laws of motion and universal gravitation.

The definition of weight includes g, which is a constant specific to the Earth. This hints at the key problem with using weight as a measure of how much matter is present. A 6 kg lump of matter on the Earth weighs approximately 60 N, but if you move that same amount of matter to the Moon, it would weigh about 10 N. Even though it is the same amount of matter, the weight is different on different celestial bodies – more on the surface of bigger bodies and less on smaller ones. The mass stays the same, but the weight changes.

If you use weight instead of mass to measure matter, your answer changes based on where you are. Technically, because gravity is weaker the farther you are from a gravitational source, your weight changes slightly when you fly in an airplane. It would change even more if you’re in space or on a planet with a different mass than Earth. Mass is a consistent measure of the amount of matter, but weight isn’t.

Mass also creates the gravitational force that defines weight, which is why it appears in the equation for weight. This is a sure sign that mass is the fundamental property of matter, and weight is a consequence of that property.