In spite of being used every day as synonymous, these words have different meanings. We usually listen to this kind of sentence: “I weight 70kg.” Mathematically speaking, and mainly in the field of physics, this kind of sentence is not correct since the kilogram is a unit of mass and not of weight.
As to make things simpler, we must say that mass is used to measure “the amount of material” that is in a certain body. That amount of material is always the same, no matter the place where that body is. Its standard measurement unit, according to the International Systems of Measures, is the Kilogram. An astronaut has the same mass independently of being here on Earth or on the Moon. It means that if his/her mass on Earth is 70 kg; it will keep on being 70kg if he/she is on the Moon.
Regarding weight there is a measurement unit used to measure the force through which a body is attracted by the planet. It is obvious that these concepts are connected, because the more mass a body has, the bigger its weight will be, since the force through which it is attracted depends on its mass. Having in mind the example of the astronaut, his/her weight on planet Earth is not the same as he/she was on the moon. It happens because gravity on Earth exerts an attraction that is superior to the Moon’s one. It is usually said that an astronaut is “lighter” on the Moon since his/her weight is smaller. In conclusion, if an astronaut on Earth weights 70 kgf (kilogram-force), on the Moon he/she will weight only 11.6 kgf.
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