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Weight and Mass

If common usage confuses the ideas of the weight and the mass of an object, these two quantitities are actually quite different.

The mass of an object is a quantity that depends solely on the nature of the object,  and not on its location. It is a number that is expressed in kilograms.

Thus, an object with a mass of 4 kg on Earth has the same mass of 4 kg on the Moon (…) or anywhere else in the Universe.

Weight, on the other hand, is a force whose intensity depends on the location of the object.

Like all other forces, weight is a vector, having a direction (…), pointing somewhere (…) and an intensity which is expressed in Newtons.


The intensity of this force is measured using a dynamometer, which balances the weight against the tension in a spring with a known stiffness constant.


The vector Weight and the number mass should thus not be confused. But it is true that these two  quantities are related by the expression: W =mg, where W is the weight and g is the vector representing the gravitational field. This  vector varies according to the place where it is measured.

Its units are Newtons per kilogram. On Earth, its intensity varies from 9.78 at the Equator (…) to 9.83 at the poles.


It equals 9.77 at the top of Mount Everest.


It is only 1.6 at the surface of the Moon.

It is therefore correct to say that our weight on the Moon is not the same as our weight on Earth (…)

… but it is incorrect to express the intensity of the force producing weight in kilograms.


As for our mass, it is the same everywhere.

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