# B Force equation: use mass or weight?

#### jbriggs444

Homework Helper
When you weight meat at the store on a spring scale, it measures force. But the scale indicates kg. Therefore, you are using $kg_f$ and $kg_m$ analogous to $lb_f$ and $lb_m$ in English units. In my book, that is not SI. SI would force the spring scale to indicate Newtons.
Essentially all measurements are indirect. In my book the quantity that is "measured" is the numeric value that a device is designed to accurately determine. A properly calibrated spring scale measures mass, not force.

#### anorlunda

Mentor
Gold Member
Essentially all measurements are indirect.
OK, bear with me. I intend to be stubborn and pedantic in this case.

We are all familiar with the number 2.2 lb/kg. That can't be lb force and kg mass.

The SI system does not recognize the kg force. The English system does not recognize the pound mass. The public has been told that in school, but they don't give a damn. The public can and will continue using pound mass and kg force because they like it. Science and engineering don't yet rule this world.

#### A.T.

SI would force the spring scale to indicate Newtons.
Why not millimeters of spring elongation? You seem to conflate two separate issues:
- How direct is the measurement of mass?
- What system of units do we use to quantify mass?

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#### russ_watters

Mentor
OK, bear with me. I intend to be stubborn and pedantic in this case.

We are all familiar with the number 2.2 lb/kg. That can't be lb force and kg mass.
I agree that a scale with modes that enable output in kg or lb is presenting a contradiction to the user and at the very least it means we should be a lot more sympathetic to the OP than was the initial response in this thread.

A spring-scale measures elongation. It then converts the elongation to force by way of the spring constant. And then, perhaps, to mass by way of an assumed g. We should not be making posters feel like they are missing something obvious when the different modes of a scale are in fact telling them very different things.

#### vanhees71

Gold Member
Science and engineering don't yet rule this world.
That's the reason for a lot of problems, but now it really gets off-topic...

#### jbriggs444

Homework Helper
The English system does not recognize the pound mass
Yes, it does. The pound mass is the standard for commercial purposes.

Back when I went to school, my physics teachers and textbooks took great pains to say that the U.S. pound is always and exclusively a unit of force. Those teachers and textbooks were simply wrong. If you see packaged goods with a label such as "net weight 1 lb 2 oz", those are mass units and that net weight refers to a mass quantity.

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"Force equation: use mass or weight?"

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