Mass and weight are similar terms, but they have slightly different meanings. Weight tells us how much of something there is by telling us how much gravity is pulling down on it. Mass, on the other hand, just tells us how much of something there is, independent of the force of gravity on the object. One might reasonably ask, why does this distinction even matter? The reason it matters is that if we went to the Moon, for example, the weight of an object would change (because the force of gravity on the Moon is different from that on Earth), but the mass would be the same, since the object is made of the same amount of stuff; we just need to make sure everyone is talking about the same thing.

We should point out that people commonly use scales to measure either weight or mass, but when we’re using them to measure mass, the measurement relies on the fact that the scale has been calibrated based on the strength of gravity on the Earth, so it “knows” how to figure out the mass of an object from measuring its weight.