## Weight and Mass

What's the difference between weight and mass?

As long as you stay on Earth, the difference is more philosophical than practical.

Uh...what do you mean by that?

Well, mass is a measurement of how much matter is in an object; weight is a measurement of how hard gravity is pulling on that object. Your mass is the same wherever you are--on Earth, on the moon, floating in space--because the amount of stuff you're made of doesn't change. But your weight depends on how much gravity is acting on you at the moment; you'd weigh less on the moon than on Earth, and in interstellar space you'd weigh almost nothing at all.

But if you stay on Earth, gravity is always the same, so it really doesn't matter whether you talk about weight or mass.

That's right...but scientists still like to be careful about distinguishing between the two. If you talk about the mass of an atom--as I will do from now on--you're always talking about the same thing; if you talk about its weight, what you mean depends on where the atom is.

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