Most metric "bathroom" scales measure in kilograms.  We commonly call this measurement our weight.  Is this the appropriate terminology?

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mwmovr40 eNotes educator| Certified Educator

Generally we are careless in the units of measure we use in our common everyday lives.  There is a difference between our weight and our mass.  A bathroom scale that measures the amount of matter on the scale in units of kilograms is actually measuring our mass.  Regardless of what we call it in common speech, it is still our mass.

As we know, mass does not change with respect to our relative position to the Earth.  Consequently, if everything else is the same, when we measure our mass in our home or on the top of Mt. Everest or the bottom of Death Valley the mass would read the same.

Of course, not all things remain the same.  For example if you step on the scale early in the morning before you go to the bathroom and then again later in the morning after you have relieved yourself your mass will change.  To say that "mass does not change" is therefore not entirely true.  It is only true to say that it does not change relative to changing position relative to the Earth.

When we say we weigh a particular value we are referring to the attractive force between the Earth and our mass.  This changes depending on how far we are from the center of the Earth.  Therefore when we step on that metric scale we really should be saying "our mass is 60kg" and not refer to it as weight.

Another interesting thing about our "changing weight" when being measured in kilograms is the phenomena of pushing up with our feet on the scale.  The mass number will increase even though the amount of matter in our body has not.  Why?  Well, when we push up we increase the force on the scale.  This force is interpreted as an increase in our weight and because our weight is the product of our mass and the acceleration due to gravity, the calibration of the scale interprets this as a higher mass.  So our mass appears to increase, but in fact it is just a result of changing the forces acting on the surface of the scale.


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