Why do scales weigh in kilograms and not in Newtons?

Answer

Hi Duncan,

You have to make a difference between the quantity you want to measure and the way in which you perform the measurement.

If you buy a kilogram of apples, what you want to know is that the mass is one kilogram. That way you will always have the same amount of apples whether you are on Earth or on the Moon.

You are right, however, that a scale works on the basis of the force that the earth exerts on the mass to be weighed. But that does not prevent us from calibrating the scale in kilograms. Because after all, it is the masses we want to know. Of course you have to remember that if you buy apples on the moon :-), you have to recalibrate the scale.

Another example would be the speedometer in a car or an old-fashioned ammeter; they also work on the basis of a force exerted on a torsion wire. But there too, the device is calibrated in kilometers per hour or in amperes, because it is the speed and the electric current that you want to measure. In fact, all measuring devices work on the basis of a force or energy that is transferred.

You see that it is very important, if you use a measuring device, that you know how the measuring device works. Because only in this way can you use it correctly and you know where the limitations of the device lie (for example: a terrestrial scale will give an incorrect result on the moon).

Greetings,

Philippe
Physicist, Iowa State University