Understanding the Meaning of '100 Pounds' in Physics

[Vorbis]

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

 

[D H]

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]

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.

 

[Vorbis]

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

 

[D H]

It depends on who is doing the talking. Everyday people typically just mean how much it weighs and don't give a bleep about the nuances. They do care if it hits their pocketbooks, for example, if the grocer's scales are crooked. Lawyers, the same. A one pound can of food had better have the same amount of stuff on it atop a mountain near the equator as it does in a depression in the Arctic. Aircraft designers are almost certainly talking about weight of some object (usually an airframe) being the gravitational force acting on the object. And finally, when you step on your bathroom scale: You are measuring yet another meaning of the term weight.