Everything in the Universe is made up of matter, which means that it has mass. When gravity is present, everything in the Universe also has weight. These two attributes (mass & weight) are similar but they are not the same thing.

Mass refers to the amount of matter in an object. Objects with more mass have more matter, or atoms. While less massive objects have fewer atoms.

Weight, on the other hand, is a measure of the gravitational force exerted on an object. While it is partially determined by the mass of the object it is not directly proportional to it. In other words, two objects can have the same mass but different weights depending on the gravity present.

To put it simply, mass is how much stuff is in an object and weight is how heavy that object is.

When recording the mass of an object, scientists use the weight (Earth weight). They do this, because it is impossible to count the number of atoms in an object. There are just too many and would literally take millions of years. We know that more massive objects weigh more, so we cheat, and just use the weight (on Earth) to represent the mass.

However, it is important to remember that mass and weight, while they do have the same value on Earth, are still not the same thing. Mass only refers to the number of atoms, while weight refers to both the number of atoms and also the amount of gravity present.

As an example, suppose that an object on Earth has enough atoms so that its weight equals 50 kilograms. We would say that both the mass and the weight of the object equal 50 kg. What would happen though if we took this same object to the Moon? Would it still weigh 50 kg?

The answer is no. Because there is less gravity on the Moon and remember that weight depends on both the mass (amount of matter) and the strength of the gravity. On the moon, the object will weigh significantly less.

However, the mass will remain the same. Because we haven’t changed the number of atoms in the object. It still has the exact same number of atoms that it had on Earth, and so the mass would still be 50 kg because again, we use Earth weight to represent mass. Thus on the Earth objects have the same value for both mass and weight, but almost anywhere else in the Universe the mass and weight are going to be different values.

Remember that mass is only the number of atoms in an object, while weight is both the number of atoms and the strength of gravity.