Basic question about mass and weight

1. Oct 6, 2008

fluidistic

I'm confused about mass, weight and kilogram force. Not that I don't know what the 2 firsts are but here I give the example that is perturbating me.
Say a home scale gives my "weight" as 67kg. I know that a weight is a force and therefore should have units of force, like the newton (N) and not the kilogram which is a unit of mass.
So what is giving me the scale? Is it my mass? Is it my weight but with the wrong units? Or is it my weight in kilogram force (unit is kgf if I recall well)? I'm not even sure that a force can be given in kilogram force, but I guess yes...

2. Oct 6, 2008

atyy

It's your weight because it uses gravity to produce the number. And it's also the right units (67 kg = 670 N), at least if we allow particle physicists to use eV as a unit of mass!

3. Oct 6, 2008

cepheid

Staff Emeritus
Just to add to atyy's answer...a weigh scale measures weight. If it operates using a spring, then what is being measured is the displacement of the spring, which is directly proportional to the force pressing down upon it. The scale has simply been *calibrated* to show you what mass (in kg) ON EARTH would be required to provide that force and therefore that displacement. In summary, the scale is displaying the mass of an object having the measured weight ON EARTH.

4. Oct 6, 2008

cepheid

Staff Emeritus
For clarity, here's an example:

If you took your scale to Jupiter, where you weigh FAR more than you do on earth, the weigh scale would show you as having a much higher mass than your actual mass. That is because it would be displaying the mass an object would have to have in order to have such a large weight on earth. The labelling of the tick marks on the scale would be quite literally incorrect (for Jupiter), and this weigh scale would be useless as an instrument for determining mass by measuring weight.

MORAL: It's dangerous to infer mass from weight unless you know how strong the gravitational field you're in is.

5. Oct 6, 2008

fluidistic

Ok, so it measures weight but shows mass.
And you also need to know how is calibrated the spring into the scale, more precisely you need to know the value of g that it has been assigned to use for the conversion weight-mass.
That mean that if one day I have to play with $$E=mc^2$$, m would be what shows the scale.
Thanks to both.