# Measuring mass

## Main Question or Discussion Point

If weight is measured in newtons, then why do we say we are 100 pounds? Do we actually mean 100 pounds of mass?

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D H
Staff Emeritus
A bunch of reasons. A couple of them:

1. We scientists and engineers often accuse the lay community of stealing and perverting our words (e.g. theory). This is a case where we scientists and engineers have stolen and perverted a perfectly good word, weight. Legally and colloquially, weight is, and always has been, a synonym for mass.

2. Pounds are a unit of mass, not force. If you mean the English unit of force you need to be specific: pounds-force, or lbf for short. Of course, turn about is fair play. If you don't want people to wonder which pound you mean, you should say pounds-mass, or lbm for short.

CompuChip
Homework Helper
Yes. There is of course a hidden conversion factor (acceleration due to gravity on earth, about 9.8 m/s^2 in SI units, I don't know it in your units - sorry) which allows us to relate force to acceleration. Of course this is not entirely unambiguous, because the exact number (9.8...) depends on where you are on earth, but in our everyday life, we all know what we mean.

So when we talk about how much something weighs, we are actually talking about how much mass it has, right?

D H
Staff Emeritus